Washington — When and if nuclear arms control talks resume, the most difficult issues may not be the size and number of bombs and rockets. Instead, any lasting form of nuclear disarmament is likely to hinge on how to verify that the nuclear superpowers are not cheating.
This is clear in the recent charges and countercharges of treaty violations flying between Moscow and Washington. It was evident during Sunday's presidential debate. And it is reflected in recent congressional activity on the defense budget.
In recent years, according to many experts, the United States and especially the Soviet Union have been nibbling away at treaty restraints. The result: Suspicions about the effectiveness of arms control have grown, along with general lack of trust.
''Hedges, past and prospective, are accumulating to the point where both sides are questioning each other's intentions toward previous agreements as well as the value of the negotiating relationship,'' warns Michael Krepon, a former US arms control official now studying verification and compliance at the Carnegie Endowment for International Peace.
''As the unraveling process becomes increasingly apparent, it also becomes increasingly difficult to stop,'' he adds. ''Both sides seek to protect themselves against worst-case scenarios by defining treaty obligations in still more-permissive ways or by exploiting ambiguities in treaty texts.''
For example, large radars being built in both countries may violate the Antiballistic Missile Treaty of 1972, as could the development of advanced antiaircraft weapons that might have use against missiles.
The Soviet Union is testing a new ballistic missile that Western analysts call the SS-X-25. Whether or not it violates the second strategic arms limitation treaty (SALT II) depends among other things on the ratio of its throw-weight, or carrying capacity, to its total weight. This has been difficult for Western intelligence sources to determine, because the Soviets are broadcasting in secret codes (encrypting) much of their missile flight-test data.
''It's one in which they're pushing very very hard against the limits,'' concedes Paul Warnke, who headed US arms control efforts during the Carter administration; he charges Soviet violations have been blown out of proportion.
Arms control advocates are not optimistic about the effect of current trends in hard-to-verify weapons on the arms control process. These weapons include sea-launched cruise missiles (SLCMs), antisatellite weapons, and mobile land-based missiles, all of which are being developed if not deployed by both sides. So-called ''national technical means'' of verification - satellites, reconnaissance aircraft, seismic sensors, and other long-range intelligence devices - may not suffice with such easy-to-hide systems. More intrusive forms of verification, such as on-site inspection, may be required.
Democratic presidential hopeful Walter F. Mondale favors a mutual, verifiable freeze on the testing and production of new nuclear weapons. But he has not been specific about how he would verify such a freeze. The Reagan administration, too , is looking for better verification measures as part of its arms control proposals.
Meanwhile, the one US-Soviet forum for discussing questions about treaty compliance has apparently not been very successful during the Reagan administration. This is the Standing Consultative Commission, which was established in 1972 as part of SALT I. The SCC meets twice a year in private and now is holding its fall session in Geneva.
The head of the US delegation is retired Air Force Gen. Richard Ellis, former commander of the Strategic Air Command. Despite his military background, General Ellis is well respected by arms control advocates. He favored the unratified SALT II agreement, which may make him suspect in the view of some administration hard-liners. For this reason, as well as the Reagan administration's general distrust of arms-control efforts, Ellis has not been given wide latitude to settle treaty questions privately with his Soviet counterparts.
''He's a first-rate guy,'' says Mr. Warnke. ''But he's been constrained by his (negotiating) instructions.''
Because they had been so critical of SALT II, Reagan administration officials - especially during their first two years in office - did not press what they called this ''fatally flawed'' treaty in SCC sessions. But actions by the Soviet Union - particularly since the invasion of Afghanistan in 1979 - have also undermined this compliance process.
The Soviets ''consistently act in ways that undermine the political relationship,'' says Mr. Krepon. For example, while SALT II allows only one new intercontinental ballistic missile, the Soviet Union has four missile design bureaus, and they all are very active.
In recognition of potentially more difficult arms control verification problems, Congress amended the 1985 defense budget in two tentative ways. It barred further antisatellite weapons tests until next March and limited such tests during this fiscal year to three. Lawmakers also passed a ''sense of the Congress'' resolution urging the Pentagon to find a way to distinguish conventional Tomahawk cruise missiles on ships and submarines from those with nuclear warheads.
A House measure that would have barred deployment of sea-launched nuclear Tomahawks unless the Soviets deployed a similar weapon failed to pass.