The telltale signs are everywhere:

*In Rockville and Bethesda, Giant Food Inc. parks a camper van with a large "Now Hiring . . . Apply Within" sign outside busy grocery stores, hoping passersby will apply for jobs on the spur of the moment.

*In Richmond, Safeway Stores Inc. recruits senior citizens to assume the hard-to-fill, entry-level posts of courtesy clerks -- those employes who bag the groceries, load them into cars and keep shopping carts in order. On the other end of the country, in San Francisco, Safeway works with local school systems to train physically and mentally disabled students to fill the same positions.

*In Atlanta, New York and a growing number of cities around the country, Burger King offers educational scholarships to lure teen-agers to work at the fast-food chain.

*Meanwhile, at Roy Rogers, employment applications frequently are used as place mats.

Perhaps the most significant sign, however, is the wages. Fast-food chains, grocery stores and other retailing companies that traditionally have paid minimum wages to entry-level workers frequently are paying far more than the current $3.35 hourly minimum wage. In Washington suburbs, many fast-food franchises are paying $4 or more an hour. And in Atlanta, where business is booming, some chains are even offering $5.50 to $6 per hour for part-time work.

What's more, many companies are boosting wages rapidly for new employes they want to keep, to make sure they don't leave for a better-paying job.

As the diverse, but numerous, signs indicate, many entry-level jobs are going unfilled -- even though 8 million Americans are now out of work.

In the Washington suburbs and in many other areas of the country -- especially in the more affluent communities -- retailers, grocers and restaurant owners are complaining that they can't find enough good employes to fill jobs they have.

One of the chief reasons: The large pool of teen-agers and young adults who have historically filled these entry-level positions is shrinking rapidly. The number of employes between the ages of 16 and 24, once one of the fastest-growing segments of the work force, is now on the decline, accounting for a smaller and smaller share of the nation's labor force.

In 1980, there were 25.3 million workers between the ages of 16 and 24, representing 23.6 percent of the work force. Last year, there were 23.9 million young adults, making up 21.1 percent of the labor force, according to the U.S. Bureau of Labor Statistics. By 1990, their numbers should shrink even further, to 21.3 million, or 17.3 percent of the work force. By 1995, they will account for only 14 percent of the labor force, with 20.2 million employes.

As a result, many employers are now scrambling to fill the growing void left by the country's changing demographics.

"There is definitely an employment problem," said David Forman, corporate vice president in charge of human resources for Peoples Drug Stores Inc. "We feel it very acutely in certain areas and for certain jobs. It is difficult to have an adequate applicant stream even to fill a job," said Forman, who noted that Peoples, like Giant and Wendy's International Inc., has started to use its television and radio advertisements not only to promote wares, but to tout employment opportunities as well.

"We attribute the problem to full employment and the changing demographic pool of employes," Forman said. "A lot of part-time positions traditionally have been held by teen-agers. There are fewer of them around, so many jobs are just going open." Labor Pool Aging

Forman said it was difficult to calculate just how many jobs were going unfilled at Peoples. However, he noted, precise numbers are irrelevant to the problem. "If two part-time jobs go a-begging in one store, that may not be very significant in numbers, but it's a very serious problem," Forman said.

"I've been in this business 25 years and have never seen anything like this," said Tom McNutt, president of the Local 400 Retail Union. In particular, McNutt noted, some fast-food chains in the Washington area are paying $4 to $4.50 per hour for beginning part-time workers.

It's not that there is no more unemployment, cautioned Audrey Freedman, executive director of the human resources program for The Conference Board, a New York corporate-research group. "With 8 million people looking for work in any one month, it's hard to believe that there is a shortage under any circumstance."

What there is, however, is a need for many industries to change their employment practices, she added. "Some of the habitual hiring practices have to change. Companies that hired teen-agers can't rely on word of mouth like they used to -- especially if they try hiring old people that are less dependent on social contacts," commented Freedman.

Although shortages occur in many cities across the country, the problem is by no means nationwide -- or citywide, for that matter. The most acute shortages are occurring primarily in the more affluent suburbs where business is booming and unemployment has declined.

"In upper-income areas, there are only so many people available in a pool to do the job," said Jerry Donovan, Giant's vice president for personnel. "Some don't want to work, and others tend to opt for higher-level jobs."

As other area employers note, Giant has no problems finding workers for the District, where the unemployment rate for September was 9.1 percent -- far above the nationwide level of 6.9 percent. But in Montgomery County, where unemployment was 2.5 percent, and in Fairfax County, where the rate was 3.3 percent, "everybody has been scrambling," Donovan said.

Despite a campaign by District officials to get employers to use inner-city workers for the unfilled suburban jobs, suburban employers say it is hard to lure D.C. workers, primarily because of the lack of good, inexpensive transportation.

Many of the jobs are beyond the reach of Metro, noted Maurice Wilson, regional coordinator for community employment for Marriott Corp. But for those jobs that are close to Metro stops, "it's the cost of transportation," Wilson said. "Many workers on minimum wage can't afford to travel from Northeast D.C. to Rockville on the Metro daily. And the bus, although cheaper, takes forever," Wilson said.

Other cities experiencing the same problems as Montgomery County and Fairfax include Boston, Atlanta and Dallas-Fort Worth.

"Part of the problem is that there is more competition for the shrinking work force," said Paul Stevenson, personnel manager for the retail division of Southland Corp., the parent company of 7-Eleven stores. "There has been a proliferation of fast-feeders, and there are a lot more convenience stores than five to 10 years ago." As a result, Stevenson said, Southland is "doing a lot of brainstorming right now, trying to become more creative and less traditional as we move into the 1990s. The basic traditional recruiting tool -- putting a help-wanted sign in the window -- is no longer going to work."

"You can run the usual ads and get nowhere," added Helen Drivas, The Hecht Co.'s divisional vice president in charge of employe relations and civic affairs.

Realizing that, Hecht's decided last fall to place an ad, not in the classified section as it normally does, but in the "Style" section for a job fair that Hecht's decided to hold just for its stores. "We thought that if you're a homemaker with no thought of going to work, you would certainly not look at the classified ads. But you might be intrigued if an ad was in 'Style,' " Drivas said.

For Evelyn Rapazzo, a former accounts-payable manager at New York's Bergdorf Goodman, that strategy worked. Having just moved to Washington with her husband to be closer to her children, Rapazzo, 60, had no immediate desire to go to work last fall. But then she saw the ad and noted that the job fair was conveniently located near a Metro stop in Crystal City. "That was it," said Rapazzo, who was hired on the spot as a part-time receptionist.

With many qualified applicants like Rapazzo looking only for part-time work, employers have found they have to be more flexible in setting a job's criteria.

"We had an opening for an accounts-payable clerk -- someone who needed clerical, mathematical skills -- to process invoices at our Springfield headquarters," noted Peoples' Forman. "We got few responses for the job when we listed it as an 8:30 to 5 job. So we shifted the hours to 6 to 10 in the evening, and got a better response." Meanwhile, both Safeway and Giant have decentralized their employment process to make it easier for applicants, permitting them to seek jobs at stores near their residences.

"They no longer have to go to our employment centers in Riverdale, Md., or Alexandria . . . " commented Don Saulpaugh, Safeway's human resources supervisor for the region. "This seems to have increased the applicant flow substantially, from 275 to 300 applicants a week to 350 to 400 a week."

In other parts of the country, Safeway is trying a host of other innovative programs, setting up a 90-day training program for disabled students in San Francisco, among other things. More than 100 of the 175 students who have gone through the program have been hired by Safeway for entry-level positions. In one case, a deaf student was eventually promoted to cashier. "He reads lips and other clerks keep an eye out for him" if there are any problems, explained a Safeway spokesman.

In Richmond, meanwhile, Safeway has been aggressively seeking senior citizens, and in one store has 10 employed as courtesy clerks.

Following Safeway's lead, other retailers are now courting senior-citizen groups, participating in their job fairs. However, one retailer noted, the fairs are not always successful, saying, "Most are looking for sit-down jobs."

Yet, senior-citizen groups reply that the lack of interest stems primarily from the low wages the retailers offer. "The problem with retailers is the small wages they are offering," said Gladys Sprinkle, director of the Over-60 Counseling and Employment Service of the Montgomery County Federation of Women's Clubs.

To entice the elderly, Burger King is offering to pay the transportation costs of some senior citizens in the Boston area. Meanwhile, to continue to attract teen-agers, the chain is offering scholarships to employes. Under the Burger King plan, any employe who has worked for the chain for three weeks is eligible to receive $200 every three months -- up to $2,000 maximum -- for any post-secondary education, ranging from beauty school to college. When the company offered the scholarship on a test basis last year, "we found that turnover rate decreased by 250 percent," a company spokesman said. As a result, the company has now rolled out the program nationwide. Cookie Firm Sweetens Offers

The Great American Chocolate Cookie Co. also offers scholarships to employes at its 300 franchises. But in order to qualify for the $1,000 sum, a worker must have worked at least an average of 20 hours a week for two years. Those requirements are not luring many workers, one franchise owner said, noting that most teen-agers who work there are not in the business for the long-term, but just for a short while to earn extra spending money.

The franchise is now considering implementing a bonus program, awarding employes anywhere from $25 to $100 for finding new employes to work at the company.

As the Cookie Co.'s president, Arthur Karp, declared, declining teen-age employment "is a problem looming on the horizon, and we want to be as prepared as we can."