When Persident Carter arrives here Wednesday, he will find a country poised for what the Mexicans unsmilingly call "our moment of truth." And, like the bull that blunders blindly out of darkness into the brilliant light of the ring, Carter himself may experience a few painful truths.

There can be no doubt that the Mexicans have every drop of oil they say they have. And more, perhaps. Nor after even the briefest acquaintance with officials of President Jose Lopez Portillo's government or of Petroles Mexicanos (Pemex), the national oil company, can there be any doubt that Mexico is determined to maximize its newly confirmed hydrocarbon wealth.

Mexico's leaders are intent upon using the country's oil not only to cure huge internal economic problems, but also to redress what they see as some serious imbalances in their relationships with the United States and other trading partners. But most especially the United States.

Carter will have to ask himself two main questions as he and his entourage hurl themselves through the series of diplomatic "photo opportunities" set between Wednesday and Friday. One question is whether Mexico can accomplish all it must accomplish with its newfound wealth. The other is more direct: What's in it for the United States?

On both questions, it can be argued that Carter's bearish public attitude is justified.

Taking the last question first, Mexico and its almost unbelievably huge possible pool of oil and gas (300 billion barrels against Saudi Arabia's 167 billion) are unlikely to fill American energy needs or to be the salvation of the American businessman.

It may well be, as a senior U.S. embassy official said here last week, that "the (Carter-Lopez Portillo) talks are not really about oil, but about immigration."

There has been an understandable tendency to view Mexico and its oil as the next gold rush -- a North Slope of Alaska, if you will, but 10 times bigger and many more times more profitable than that. Certainly profits will abound there, and business aplenty, as the British, French, Germans, Japanese, Spanish and Yugoslavians are finding out.

But the reose-colored scenario that some observers have constructed may have its darker side, just as one need wander only a few blocks from the luxurious Zona Rosa -- the fabled Pink Zone of designer boutiques and fancy hotels -- to see the real Mexico City, a choking, 13-million-person powder keg of rural poverty come to town.

The first roadblack an American businessman must confront if he wants a piece of the Mexican action is the Mexican government itself. Here, 1930s-style socialist rhetoric paradoxically blends with oligarchic familybased power elites. The combination produces an infuriatingly slow business pace that can flare suddenly into the slickest sleight-of-hand before the foreign visitor wakes up.

The bottom line is that Mexico wants it all.

The American businessman, his London cousin and his Tokyo arch-rival are all welcome in Mexico. But the businessman trying for major contract must be prepared to build his plant in Mexico, use mexican labor and raw materials, and -- this is where Carter may be in for a shock -- be prepared to export some of his products to the United States. All this -- and with a Mexican partner who by law must own 51 percent interest in the operation.

Undeniably there are boom-town stories of rich contracts held by Americans down here. One construction company building a resort on the southern Gulf Coast is flying its workers into the site each day and out again at night because of the lack of suitable housing. The point is, the storyteller insists, the builder is still getting rich. But then there are other cases.

Case One: British oil industry officials have watched with glee over the past year the visible fraying of U.S.-Mexican energy relations. The North Sea will be "maturing" as a developing project in a few years, and the British need new technological worlds to conquer to keep their industry alive.

Two weeks ago, Dickson Mabon, the top civil servant in the U.K. energy department, left Mexico City claiming an agreement was reached on "hundreds of millions of pounds" worth of orders for British oil tool, technology and engineering firms.

Mabon said the deal was all the more a coup because it also involved a swap of some of the lighter North Sea oil for Mexico's heavier crude product that is needed badly to mix in Britain's refineries. Better still, there were even talks started regarding U.K. engineers helping Mexico develop its still-infant uranium industry, with an eye to Britain using Mexican ore to replace lessthan-certain supplies from South Africa.

Against this triumph last week, executives from 35 corporations belonging to the Offshore Centre arrived here with order books at the ready. The Centre is the London-based trade group of all the major U.K. and European firms active in North Sea development, ranging from British Petroleum and Weir Pumps to Rhine-Schelde-Verolme and Royal Negher-lands Harbour Works. Instead of orders, however, the officials received a harsh dressing down from Pemex Director Jorge Diaz Serrano.

"They were plenty taken aback and quite shaken by the experience," a London banker who met with the group said later. "The surprising thing is why Mabon said what he did back home. The Mexicans are sensitive about buying technology and even tools when the policy is to make them domestically. We could find ourselves left out of the game if we're not careful."

Pemex officials are still bitter about the experience. Jose Felipe Ocampo, a deputy to Serrano, grew quite indignant days after the event. "We have always made it clear that Pemex policy and government policy are the same. We do not want to sell oil abroal only -- like Venezuela. We must build jobs, create industries and we want to deal with firms which want to help us do that. I can't imagine why he (Mabon) said that."

Another official was more acid. "Does he (Mobon) think we don't read London newspaper? Does he think he is James Schlesinger?"

Case Two: Bracketing Carter's visit here just a week later is the arrival of French President Valery Giscard d'Estaing. The French have done rather well in Mexico, having won a 100,000-barrel-a-day export allocation, and Giscard d'Estaing is planning to get still more. He may not get it.

"The French have an advantage over the British and other foreigners. They are working on several big-number projects such as the original construction, and now the enlargement, of the Mexico City Metro subway system," an American banker explained. "The British have a few factories here -- mostly diesel works, a steel ill and other high-technology stuff. The French actually put Mexicans to work, and the counts."

For how much? How far can Mexico be tempted to pump up its oil exports to pay for hardware -- and even jobs that produce hardware?

"It still counts for a lot, but less and less all the while," the banker conceded. "Right now, the favored foreigners here are the Spanish since they have offered a deal to build a refinery for Pemex oil in Spain and sell the product not only in Spain but throughout the Common Market, for substantial hard-currency profits."

Case Three: Mexico's "Bolsa," its stock exchange, is a tiny band-box affair with fewer than 500 listenings, aonly a handful of which are available to foreigners. Yet the stock exchange's index has trebled in the past year and the average share's value has risen 185 percent. The flood of foreign capital trying to buy its way in past the government's ownership restrictions grows daily.

This is the clearest case of an apparent conflict between Mexico's needs in 1979 and the old nationalistic prohibitions that keep foreign owners of Mexican companies to a 49 percent minority total. But is the conflict real?

"Certainly not," said Gustavo Petricioli, who heads Mexico's equivalent of the Securities and Exchange Commission. "We want that capital, we need it and so we are liberalizing the limitations on foreign investment in our stock market and we are encouraging more Mexican companies to issue a class B stock specifically for sale to foreign buyers."

Class B stock. That means non-voting stock?

"Yes, of course," Petricioli answered. "We will welcome foreign capital which is interested in the 30 percent yields most stocks can easily produce and the fact that we have no capital gains tax on share profits. We don't want anyone interested in seizing control. We are not changing our law on this matter, just the interpretation of it."

And so it goes. General Motors Corp., Ford Motor Co. and Chrsyler Corp. all have reproted booming sales in 1978 from their Mexican subsidiaries.While these are totally Detroit-controlled, their de jure ownership was worked out with the government.

But Caterpillar Corp., which has had a parts manufacturing plant here for years, recently incurred the government's wrath because it declined to take a Mexican partner and build a heavy-equipment construction plant as well. A Japanese competitor signed the pact and the big Cat earthmovers now face duties that will leave them under a competitive handicap.

What this means is that an American businessman or investor has to use a very sharp pencil and all of his wits when he seeks a piece of the Mexican action. And finally, like Carter, the businessman must come back to the first question that was asked about Mexico's future: Can Mexico make it into the 21st Century? If not, the shortterm profit picture doesn't justify a long-term investment.

How short? How long? One constantly ignored fact of life is that successive Mexican presidents may come from the same political party and are possibly during their terms the closest thing to an absolute monarch there is.

But in 1982, after six years in office, Lopez Portillo will be unable to hold office again. While Diaz Serrano was handpicked to take over the important Pemex post, he may not be able to succeed Lopex, and thus the important continuity of government policy may shift in 1982 just as sharply as it did in 1976 when the Echeverria government's free-spending public works programs were halted abruptly.

More ominously, the economic numbers thrown about by government officials and the press here and in the U.S. do not really add up to much of a future for Mexico. And there is some doubt about the validity of the numbers themselves.

Unemployement is generally conceded to be at 50 percent of the work force and swelling as 800,000 new job seekers enter the economy each year. That says nothing about the underemployment of those Mexicans who do have jobs or the unproductive work rules of the country's powerful unions representing 30 percent of the work force. It does not say that some parts of Mexico are outside the economy (and often outside government control) altogether.

Worse, the government's inflation rate figures (18 percent in 1978) generally are held suspect by foreign bankers here. But finally, there is the increasingly voiced conclusion that Mexico simply cannot pump enough of its vast oil holdings to create enough jobs for its 66 million people, who will become 130 million people 21 years from now.

"It is simple and undeniable," the senior American embassy aide told me. "Mexico will have its hands full to pump the 2.2 million barrels a day Lopex has set as a limit by 1982."