President Reagan has just gone on record against a kind of East-West detente he saw as ''only a cover'' for Soviet military buildup. Meanwhile, his State Department is wisely trying to make the most of a prime, though often maligned, legacy of detente in a broader sense. This is the 1975 Helsinki declaration on human rights and European security and cooperation - and its unique ongoing forum for dialogue on differences in philosophy as well as conduct.
The current venue for the United States, the Soviet Union, and the rest of the 35 participating countries is the Helsinki review conference in Madrid. It has been proceeding on and off since November 1980. As it grinds toward a possible conclusion, participants are contemplating two spin-offs: a Stockholm conference on reducing military tensions in Europe and, farther in the future, an Ottawa conference of human rights experts.
Yes, propaganda and stalemate often seem to be the order of the day. Yet the Helsinki declaration at least enunciated standards and goals against which the signatories could be tested. Helsinki ''watch committees,'' often at great risk to themselves, have helped to keep at least some attention on grievous rights violations in Eastern Europe and the USSR. And their riskless Western counterparts have blown at least a small whistle on the democracies, too, when these have fallen short of Helsinki commitments much closer to their ideals than to Moscow's.
At the same time, Helsinki has continued to serve European security in the sense that its participants continue to talk rather than fight. Amid all the wrangling in Madrid, much of a final document, such as a long section on trade and technology, has not been contested. Even on vexed questions of rights there have been encouraging signs of conciliatory progress toward Helsinki goals.
The West, for instance, has been willing to defer some earlier demands in the interest of achievable results. A group of neutral and nonaligned countries has proposed a document considered 80 percent pro-Western. For example, it replaces vaguer language on processing exit permits ''without delay'' with a provision specifying that an application be replied to within six months. Moscow, notorious for delaying exit decisions, has accepted this document, though with at least an initial adamant refusal to consider changes still considered important by the US and its allies. These include a call for a meeting to deal with separated families, a clarified mandate for the Ottawa conference due in a couple of years, and a stress on international broadcasting in relation to a free flow of information.
The last point provides an instance of Helsinki-style nuances. Earlier the US had demanded an end to radio jamming by the Russians. The new version does not mention jamming, yet it must imply a contradiction between jamming and free information.
As for the Stockholm meeting, derived from a French call for a disarmament conference, it actually is supposed to be limited to confidence- and security-building measures, with the possibility of a second phase to do with disarmament. Tentatively scheduled to begin Nov. 15, it would seem a convenient occasion for Moscow to fulminate against the December deployment of US missiles in Europe. But Moscow has shown it does not need any special occasion for fulminating. (Moscow, of course, has it in its power to prevent as much of that deployment as it wants to by agreeing to ''zero'' or other mutual limits on such missiles.) And Europeans, notably the West Germans, would like to have the conference - not to mention success at Madrid - to show the public that dialogue remains possible despite the arms race in their midst.
But America's Madrid negotiators have to go back to a country particularly dubious about all talk and no action when it comes to Moscow. Here is where Moscow could ease Madrid's credibility problem in the US - by giving the conference something to show for Helsinki in addition to a document as strong as consensus permits. Deeds, in other words.
Not that the Kremlin leaders are likely to cry mea culpa and begin feeding caviar to dissidents. But they could let it be known privately that their intentions are to follow constructive tendencies. And Madrid partici-pants could say, without necessarily citing cause and effect, that certain Soviet-bloc actions have coincided with the review conference - easing of martial law in Poland, easing the Pentecostalist case in Russia, turning back from harsh exit taxes on emigrants from Romania. It will be remembered that, for all the cynicism about Helsinki in 1975, it was followed by such steps as improvements for Western journalists in the Soviet Union.
Perhaps cynicism about Madrid is worth keeping in abeyance, too.