IN ANCIENT TIMES a traditional method for forecasting the future was to cut open a sacrificial victim and look for the omens in the viscera or entrails. The ancient practice has now made its reappearance in the Reagan administration, during the testimony of its chief forecaster, Murray L. Weidenbaum, chairman of the Council of Economic Advisers.

Rep. James R. Jones, chairman of the House Budget Committee, had sent Weidenbaum a letter asking 16 questions about the assumptions which lay behind the administrations's rosy predictions for the fiscal years 1981-1986.

The Budget Committee chairman wanted to know, first of all, whether the administration had "an econometric model" and whether the model was based on "internally consistent macroeconomic equations empirically estimated from economic history or how was it derived?" The committee asked for the basic assumptions and their supporting documents so they could be reviewed.

Weidenbaum appeared before the committee on March 11, the day after Reagan sent his final budget and tax message to Congress. But neither the president's message nor Weidenbaum had the answers. When pressed by Jones, Weidenbaum finally pleaded that he was relying in part of his "visceral computer model . . . the guts feeling of a forecaster." We are back to entrails.

Weidenbaum pleaded for faith in his 30 years as an economist. But there is no way to feed gut feelings directly into a computer. Every economic forecast is based on certain assumptions. The assumptions are fed as mathematical equations into the computer. The results are then matched against the facts of experience.

If they don't match, there is something wrong with the assumptions. One way to avoid such unhappy conclusions is to tinker with the known facts. These are the hidden entrails into which Congress and the country have a right to look. To withhold the nitty-gritty is to invite the suspicion that the administration fears exposure or ridicule.

The administration is understandably edgy about its budget estimates, because so few economists of any persuasion agree with them. When the president was asked last Tuesday about a Congressional Budget Office report which said he may have underestimated 1982 spending by up to $25 billion, Reagan dismissed the CBO figures as "phony." Then, correcting himself, he said more politely that it was a question of differing assumptions.

But the administration's assumptions are exactly what Congress wants to examine. The House Budget Committee tried without success to get them out of Weidenbaum, so it could inspect them for itself and check them against economic realities. The Reaganites claim, however, that these realities no longer matter, that this is a New Era, that they have a new magic in supply-side economics that can perform miracles. But this new faith has made few converts. On the same day that the CBO issued its report, a staff study from the Republican-dominated Senate Budget Committee and a report by the Shadow Open Market Committee, a prestigious group of private monetarist economists, also foresaw less growth and greater deficits ahead than Reagan has promised.

Indeed, last Sunday's New York Times Magazine disclosed that Weidenbaum himself was privately skeptical of the figures Jones has been demanding. An article on budget director David Stockman gave Weidenbaum's original reactions to the Reagan team's assumptions. It quoted him as telling Stockman's computer brigade, "I'm Peter Pan. I want to believe. But you have to convince me, and you haven't done that."

The silences in Weidenbaum's testimony were more telling than his answers. The biggest inflationary threats ahead lie in energy and food prices. One of the House committee's unanswered questions asked "what changes in food prices" had been assumed.

Equally important in judging the administration's optimistic promises on inflatioin is to learn what assumptions went into the computers on energy. What was assumed about the effects of Reagan's accelerated decontrol of oil and natural gas prices?

A useful reminder of the magnitudes involved may be gathered from the admirably sober and perceptive "white paper" circulated a week ago by House Majority Leader Jim Wright of Texas as a Democratic rejoinder to Reagan's two economic messages.

Wright's "white paper" said that "the most remarkable thing about President Reagan's two speeches on the economy is his total silence on the subject of energy." Wright provided some striking figures to show the relative inflationary impact of federal deficits and oil imports.

The total of federal budget deficits during the last four years did indeed rise by $18 billion over the total for the previous four years. This slow creep calls for remedial action. But fossil fuel energy imports were a far greater inflationary strain. Their cost over the same period rose during the past four years by $130 billion over the previous four years. As an inflationary stimulus, that was at least seven times as great as the rise in federal deficits.

The cost of energy imports was $95 billion in the four years beginning in 1973. They rocketed to $225 billion in the past four years. The drain on the dollar and the impact on every area of the economy was immense. Yet, as Wright objected, Reagan wants to cut back on a wide variety of alternative energy programs, from solar to geothermal, designed to make the country less vulnerable to such price gouges from abroad.

What were the energy cost assumptions fed into Reagan's jolly computers? On this, too, Weidenbaum was not forthcoming.

The day after Weidenbaum's appearance before the House Budget Committee, it heard from three of the most respected economic forecasting institutions in the country: The University of Pennsylvania's Wharton School, Chase Econometrics and Data Resources Inc.

All assumed enactment of the whole Reagan program, but none saw an end to inflation. Where Reagan promises a deficit of only $22.8 billion in 1983, the Wharton School comes up with a deficit of $64.8 billion, almost three times as great.

Where the administration promises a balanced budget with a half-billion-dollar surplus in 1984, Chase Econometrics foresees a continued deficit in that year of $32.9 billion.

Where Reagan promises three successive years of balanced budget in '84, '85 and '86, Data Resources estimates continued deficits in those three years aggregating $153.1 billion -- and two years later, in '88, an all-time high record deficit of $101.5 billion.

So what if, after a series of whopping tax cuts for the truly unneedy and four years of suffering for the poor, crippled public services and social turmoil, the country finds itself deeper in inflation than ever? That is why Congress has a duty not to be stampeded. This is too big a gamble to be taken on Weidenbaum's entrails.