Let's say you want to know what's happening among the Apostolic Christian farmers in the central Illinois Corn Belt, how they are abandoning many of the customs of their Germanic ancestors and what that means to agriculture.

Where do you get a quick answer? From Calvin L. Beale. He's been there.

Or you want to know more about reports of a population boom in the historic California Gold Rush villages, places like Placerville and Muckalummy Hill. Or you need to know about similar booms in Maine and New Hampshire and what they mean to rural development programs.

Where do you go? To Calvin Beale. He's been there.

Tall, lean, professorial Cal Beale is the Department of Agriculture's chief demographer. His official job description since 1963: head of the Population Section of the USDA's Economic Research Service. People inside and outside government who know Beale and his work describe him more simply as "a national resource," a reflection of the esteem that his maps and reports command.

Before you wonder why in the world the USDA needs a population studies team, consider this: many of the department's programs have nothing to do directly with agriculture. They deal with rural housing, clean water and sewers, business development, nutrition and the like.

And if policy-makers don't know exactly who they're making policy for, we're in bigger trouble than anyone imagined. Beale's job is to provide the heavy thinkers at the USDA with those answers. If their policies don't mesh with Beale's findings, well, that's another story.

"The aim of this work is to avoid mistakes," Beale said the other day. "People have notions of the trends in society and they hold to them until they are told different."

Recently, a secretarial task force at the USDA called on Beale to provide insights on black farmers and their special needs. Earlier, as the Reagan administration was readying a rural development policy, Beale's maps and data sheets were called into play.

But perhaps the sharpest example of demographer Beale's work is the turnaround in rural outmigration, a change that has big implications for local governments, for business development, for school boards and social-service agencies.

Beale and his co-workers in the early 1970s spotted that change at a time when, as he recalled, "it was taken as gospel that there was a net loss of population from rural areas . . . this new information changed the way people thought about rural areas. The 1970s trend was under way when demographers, members of Congress and secretaries of agriculture still were basing their program proposals on data that was out of date."

That change, of course, continues apace, although it does not necessarily mean that there are or will be more farmers and farms. It means that more and more Americans, fed up with rat-races or whatever, are moving to the country. And it means serious change in rural areas, affecting business, the makeup of state legislatures and the Congress, tax bases, the demand for services.

"We are not going back to a net outmovement from rural areas--at least for the next several decades," Beale said. "There is a component of people who just want to get away from things; there are people with strong ideologies who want to change things moving into these rural areas . . . . It's very real, very pervasive . . . . People with that outlook are bound to have an impact on the way things are done in these areas.

"And you know, this is an international phenomenon. Rural and small-town populations are reviving in Northern Europe, in Canada, in Japan. It transcends international boundaries, reflects modern societies. It isn't just happening here. It's like jeans and rock music."

Beale, 60, a Washington native, studied geography at the old Wilson Teachers College here and took a master's at the University of Wisconsin. He was a Census Bureau statistician from 1946 to 1953, when he joined the USDA. He is hard on himself and fellow demographers for what they don't see.

"Demographers don't do a very good job of predicting," he said. One of the problems is that "you can't know what in hell's going on in the country from behind a desk in Washington. This department ought to require all of its employes to go out and see what is happening."

But the department doesn't. So Beale devotes his vacations to traveling around the country and touching first-hand the demographic phenomena he spots in the tables and numbers that flow over his desk.

"With the restricted travel budgets around here, there's no way the department will send me out just to roam around for a week or so," he said. "So I go out on vacation a couple of times a year to areas I don't know . . . visit USDA people, local officials, farmers, chambers of commerce, what have you. I'm trying to determine what's happening and why."

Calvin Beale's been there.