Standing amid the opulence of his Tiberio Ristorante, Giulio Santillo was trying to explain the difficulty in distinguishing who is who and what is what when dealing with immigrant workers.

To make his point, he called over a Central American who was vacuuming the restaurant floor. "He has worked here two times," said Santillo, who asked the man in Spanish what his name was the first time. "Raul Amaya." And now? "Francisco Hernandez." Both laughed.

Santillo makes no secret of the fact that he was an illegal immigrant 3 1/2 years ago when he opened Tiberio Ristorante at 1915 K St. NW. "Don't make a mistake, I came first class," he said. But not until he had been in business seven months did he obtain the "green card" that allowed him to work legally in the United States.

No secret either is that, from hot dog stands to haute cuisine, an increasing number of workers in Washington restaurants come from overseas or south of the Mexican border. Many come illegally.

Ten years ago, dishwashers and busboys, salad makers and assistant cooks - the lowest paying restaurant jobs - were held mainly black Americans from Washington and the South. Today, they are likely to come from E1 Salvador or Bolivia, or any of 100 other developing nations.

It is impossible to say how many are here. In 1977, the U.S. Immigration and Naturalization Service registered almost 14,000 aliens in the District of Columbia, more than 50,000 in Virginia and about 60,500 in Maryland.

The number of illegal immigrants is thought to be considerably higher, but the only available statistics area for the relatively small numbers who are caught. Almost 6,000 illegals were apprehended in Maryland, Virginia and the District of Columbia during fiscal year 1977.

In a normal month, according to INS District Director Joseph Mongiello, immigration investigators visit from 12 to 15 restaurants in the area searching for illegal aliens and most often find them.

Whatever the exact numbers, the obvious presence of so many foreign workers - especially so many here illegally - infuriates many U.S. citizens.

"They still don't have enough jobs for even the citizens and the legal people who are here," local political activist Calvin Rolark told a hearing before the City Council's Committee on Employment and Economic Development earlier this year. Rolark said he worked his way through college waiting on tables. Now, he said, when he enters a restaurant, "people don't go to Bethesda to work as a busboy even understand what I'm saying. If they are illegal aliens, let get them get the hell out of the country."

William Larkin of Anacostia told because Spanish-speaking people have it locked up here in Washington." He left the hearing amid a shouting match.

The bill that was being considered then and still in committee would make it a crime to hire undocumented workers in the city. Several states including Virginia, have passes such measures, and a similar federal law proposed by President Carter also remains in committee. Polls indicated that as many as three-fourths of U.S. citizens would support such laws.

The legislation is opposed almost universally by Hispanic and other ethnic groups who fear potential discrimination against anyone legal or not, who has a foreign appearance or accent. Both Congress and the City Council are far from passing such laws.

Meanwhile nowhere is the increasing number of foreign workers more evident than in restaurants serving, among other people, the nation's lawmakers.

Already about 20 percent of the membership of the Hotel and Restaurant Employees Union, Local 25, consists of people from foreign countries, according to executive secretary Ron Richardson. He said he thinks that most of them have green cards but that their presence in the union indicates the industry's employment trend.

"The lower paid jobs are more and more being taken by foreign workers rather than black Americans," Richardson said. Blacks in D. C. aren't always willing to settle for the lowest paid jobs any longer. The blacks have realized that there are ways to advance in society, and they don't want to take a dead-end job."

Richardson and city restaurant owners often have serious differences, but on that point they appear to agree.

"Who wants those jobs" asked Dominuque D'Ermo, owner of Dominique's Restaurant at 1900 Pennsylvania Ave. NW. "Blacks are not interested in scrubbing floors and doing those dirty things. But somebody's got to empty the garbage, and you don't smell like Chanel No. 5, you know."

Many employers feel that foreign workers are more diligent, trustworthy and reliable than U.S citizens who apply for the lowest paid jobs.

"They have a different concept of things," D'Ermo said recently as he sat in the dark, wood-and-stained-glass elegance of his restaurant's dining room. "They live in a different jungle. We live in a jungle of vices. They live in a jungle of fresh air and poverty. They come here and they see running water and they get a paycheck every week and they think this is paradise."

While D'Ermo had a drink with some of his patrons a 41-year-old Salvadorian, who said he has no green card, was scouring pots under the bright lights of the white-tiled kitchen.

Why had he come to the United State! He looked surprised that anyone should ask. "To make more money. I can't raise my family in El Salvador," he said through a waiter who acted as interpreter.

The dishwasher said he had tried in vain for most of his life to support his wife and 10 children on the $2 he earned each day in Central American cotton fields.

He decided to come to the United States but was not eligible for an immigrant visa since he had no special skills. Finally, he paid a smuggler, called a "coyote," $800 for passage here. He took a bus to the Mexican border and one night was led across the river to Laredo, Tex. From there he went to Houston and took a mid-night flight to Washington, where he has worked ever since.

At Dominique's, he said, he can earn about $460 a month, plus meals. He said he lives with several other Central Americans in an apartment near Columbia Road NW. He said that now he can send about $300 a month to his family in El Salvador for food and clothes.

Like other workers in his situation, he no longer must fear for his family's survival.His greatest worry is that he will be caught by "La Migra," the Immigration and Naturalization Service, and deported.

Since for the moment, no legal sanctions can be taken against employers of undocumented workers, the threat that "La Migra" poses for Washington's restaurant owners largely is an inconvenience but one that some are unwilling to risk. The manager of the Monocle Restaurant at 107 D St. NE, for instance, hires a number of foreign workers but said he always checks their green cards. As he put it: "The Immigration walks into your place and takes half your help - that is a penalty, too."

Other restaurateurs are considerably more casual.They say they are glad to have anybody who will do the work for the $2.80-an-hour minimum wage and simply do not bother to check immigration status.

"I never even thought about it," said Bernard Gorland, owner of the Sans Souci at 726 17th St. NW. "I hear about those raids on other restaurants, but I guess an accident is not an accident until it happens to you."

Even restaurants that have been raided, some of them several times, often continue to hire undocumented immigrants.

"It's no use to ask them if they have green cards," Dominique D'Ermo said. "You know they don't have any."

Others resort to tactics that avoid the question. "Someone comes to me with a dusky skin and a Spanish accent, and they are very nervous," recounted one owner of a well-known you are from Puerto Rico (and thus a U.S. citizen). And there is this great sigh of relief between us. 'Si. Si. I am from Puerto Rico.'"

In such ways are restaurant owners and their employes caught in a web of half-truths and deceptions dictated by attempts to avoid immigration problems, while obeying other laws.

Employers are required, for instance, to obtain Social Security numbers from their workers. But some, such as Giulio Santillo at Tiberio, settle for any numbers their employes give. "What they do sometimes is take a telephone number and add two digits," Santillo said.

On other occasions, they may have bought forged cards or simply borrowed cards from friends. "I demand to see the Social Security card," one Georgetown restaurateur said. "And I say, 'is this your name?' And of course he will say 'yes' whether it is or not. And I say 'okay' that is what I am going to call you as long as you are here."

Many restaurant owners feel they are compensated for problems that arise from hiring foreign workers by the devotion and attitude of such employes, although that loyalty may arise from insecurity.

"It's a question of respect for the job, for the place where they work," said Barnard Gorland of Sans Souci. "I have one Spanish-speaking guy whose wife called this morning and said, 'Oh, my husband is so sick and can't come in.' She offered to come in herself. With a black guy, you might be lucky to get a call.

"I'm not putting down blacks," said Gorland, who said he employs several blacks, "but many of them, they work here and there for a few weeks or months, just long enough to get welfare."

Even after a kitchen staff is reduced by an Immigration raid, some employers have no difficulty replacing the arrested workers, because the workers' friends show up almost immediately to apply for the vacant jobs.

Dominique's was raided a few months ago and "within one hour," D'Ermo said, "a whole new batch of people were waiting at the door."

Some restaurateurs feel a moral obligation to help their arrested employes fight deportation, others do not make such efforts. But even deported persons often return to the United States, sometimes going through the cycle twice or several times.

Occassionally, owners said, their deported employes are back on their deported employes are back on the job within a month after having told their friends about the wonders of the United States. "You send one back," a downtown restaurateur said, "and three of them are coming in."

Many workers and employers believe that subterfuges and communication networks existing now would stifle effectively any efforts to enforce proposed laws against hiring illegal aliens.

The Virginia law has been in effect since Jan. 1, but no cases have been prosecuted, according to the state Department of Labor and Industries.

Some employers who said they try to obey the Virginia law contend that they face serious problems.

Ulysses Auger II, director of operations for Blackie's House of Beef and a dozen restaurants his family owns in the metropolitan area, said he had to dismiss 40 percent of his daytime staff at the Black Crystal restaurant in Crystal City when the law went into effect.

"I've geen getting ulcers over there," he said. "The turnover has increased 500 percent. My manager asked one guy (am American) to wash a wall, and he pulled a knife on him."

Proponents of "employer sanction" laws argue that employers prefer illegal immigrants because they are so vulnerable to exploitation. As a result, their presence depresses wages and keeps U.S. citizens, particularly unskilled young people, unemployed.

Whether such laws, if or when they are enforced, would offer any solution to U.S. employment problems is not yet clear.

As the nation grapples with what President Carter has called "one of our most complex domestic problems," immigrants continue to come. In some countries, such as El Salvador, they have heard that Washington is the most promising city in the promised land.

"Here they have more work, move opportunity. In restaurants in Washington, they pay more than anywhere," a Salvadorian busboy said.

If deported, he said, he almost certainly would return. Already, he said, he has saved the money for the trip. He would take another job, perhaps to try to work his way into a chef's position. Then he might be able to get legal resident alien status because federal law permits the issuance of green cards to some illegal aliens who hold jobs, such as chef, that few Americans appear to want. The busboy looks forward to a future that, for all its dangers, seems brighter than anything he has known.