Jimmy Carter said the other day, off the cuff, that there's a "growing concern" in this country for Africa. He suggested that the nation's sense of the "many compatibilities" of the black struggles in Africa and America have deepened. But is this so? And in any event, how does he know that concern on a given foreign-policy issue is going up or down?

For a country still shadowed by the Vietnam era's split between official policy and public feeling, and for a President who has made rather a fetish of demystifying international affairs and building a foreign policy on popular consensus, these are live questions.

The answers are elusive. On Africa, for instance, there is some superficial evidence -- the Andrew Young appointment, congressional blocking of Rhodesian chrome imports -- of "growing concern." But few people who aren't already committed to an activist anti-South Africa or anti-apartheid policy detect much of a groundswell. Uncertainty allied to confusion, more than concern of a sort that impels policy, seems the dominant mood.

But that is just my seat-of-the-pants impression. If one looks for more solidly based material, the pickings are slim. None of the major national polling organizations has done much with African issues, even though their growing topicality (there's no argument about that) and the "Roots" phenomenon would appear to offer good reason.

The Foreign Policy Association, whose members are probably better informed and more interested than the public at large, recently questioned participants in its sourthern African study groups. Some 2,327 people responded, and they showed little enthusiasm for trying to alter the status quo by means other than long-distance diplomacy. Fifty-one per cent, for instance, would "work closely with South Africa to maintain stability in southern Africa"; only 10 per cent would sever ties with Pretoria as long as it maintains apartheid.

To be sure, "public opinion" doesn't and shouldn't translate directly into foreign policy the way it does much more, and much more legitimately, in tax policy or energy policy. There is not the same impact on daily existence and there is not the same legislative gamut to run. Special interests dominate public debate on specific international issues. Each such interest customarily identifies itself as a valid representative of American opinion as a whole. A President has got to take a broader view.

Jimmy Carter, in theory at least, is strong on consulting the public, although he has yet to raise the question --his view and the public's view of the United States' international requirements were not to coincide. In practice he's been more attentive to the congressional politics of foreign policy than to the broad public politics -- perhaps the latter is for an election year.

Cyrus Vance, before becoming Secretary of State, supported a Public Agenda Foundation voter-education project that, on foreign as well as domestic issues, consulted the public as well as various specialists in order to arrive at some measure of higher insight. But as Secretary, Vance has not found or made similarly systematic occasion to plug the public in. His public-affairs adviser, Hodding Carter, expressing the administration's general concern, notes that the point must invariably be "to let us know what the concerns of people are" -- not to invite the public to shape policy.

The State Department has a small "public opinion analysis" office that inspects what polls come along, newspaper editorials, and the like. But it does no polling on its own.The feeling is that if it did, Congress would object --and properly so -- that the department was trying to manipulate public opinion or cook the results of its research.

The irony is that no sooner does an administration professing respect for the public's international views arrive in Washington than, coincidentally, the leading foreign-policy opinion research outfit finds its foundation funding drying up. For six years the Washington-based Potomac Associates has been publishing respected and widely consulted surveys that measure trends in public attitudes and that illuminate feelings toward particular problem areas. It produces a State of the Nation study every two years and a Policy Perspective series.

Polling is now an accepted tool for dealing with and understanding a host of domestic problems. In foreign policy, the need for an open, informed connection between the leadership and the people is in some ways even greater, because there are fewer alternative channels of feedback. Skilled politicians can learn a lot by their own listening. Sensitive polling can perhaps tell them more.