President Reagan's report on suspected Soviet arms control violations - sent to Congress this week - gets to the heart of the way arms treaties are checked for compliance. So, too, does the apparent Soviet offer to allow on-site inspection of proposed East-West troop reductions in Europe.
Experts at all points on the political spectrum agree that this can be the knottiest part of lessening military confrontation. ''If there's one area where arms control lives up to its reputation for difficulty, it's in verification,'' says William Kincaide, executive director of the Arms Control Association.
Finding paths to mutual trust based on clear proof could be crucial to the future of arms limitations, it is also generally agreed. The new trends in weapons development - mobile intercontinental and medium-range ballistic missiles, low-flying cruise missiles, chemical weapons, rockets and artillery that can be tipped with conventional explosives or nuclear warheads, and satellite-killing rockets - all point to tougher times ahead for verification.
It would be difficult to verify such new weapons without allowing officials from one country to inspect facilities in another. The current methods of verification - the satellites, radars, and aircraft that make up ''national technical means'' - are not able to detect such weapons.
Soviet noncompliance, the President said in his letter to Congress, ''calls into question important security benefits from arms control. . . .''
The administration charges that the Soviet Union has ''probably'' and in some cases ''almost certainly'' violated several treaties involving missile testing, underground explosions, antimissile radar, chemical and biological weapons, and troop movements.
Conservatives who have been pressuring the White House on the issue say they are pleased that the administration has finally spoken out. But they also charge that there are many more Soviet violations that need to be publicized.
Arms control advocates, on the other hand, say the mechanism for discussing alleged strategic arms violations - the bilateral Standing Consultative Commission (SCC), which meets out of public view at least twice a year - should be the proper forum for quietly settling questions raised by either side. But just how well has the SCC worked? Some say it is one of the few success stories in arms control.
While acknowledging that it is a ''delicate process of groping and learning, '' a report by the Center for Foreign Policy Development at Brown University concludes that ''the SCC has been able to deal with potentially volatile matters in a timely and effective manner.'' The report's authors, former SCC commissioner Robert Buchheim and Brown University political scientist Dan Caldwell, note that the confidentiality of SCC proceedings has been particularly valuable in answering questions raised by both sides, and they assert that ''fiery rhetoric would be of no help at all.''
Others charge that the SCC is merely a tool for Soviet obfuscation and delay. In a study outlining many alleged Soviet violations of the Strategic Arms Limitations Treaty (SALT), former Central Intelligence Agency analyst David Sullivan says, ''Soviet noncompliance with treaties is now indisputable.''
The President's report to Congress this week also points up the fact that several key treaties (those dealing with chemical and biological weapons, for example) have few, if any, ways of adequately addressing compliance issues. Some experts also note that the United States has not ratified certain treaties, including the threshold test ban and peaceful nuclear explosion treaties, and that this makes verification more difficult.
''If those two treaties had been ratified, we'd now have in place a joint consultative commission clearly patterned after the SCC,'' Mr. Caldwell says. ''And we'd have a place to raise those charges and discuss them in a quiet, businesslike manner.''
And in a recent analysis for the Roosevelt Center for American Policy Studies , Michael Krepon, a former US arms control official, says: ''Without question, compliance issues have become far more serious since SALT II ratification was shelved after the Soviet invasion of Afghanistan.''
NATO and Warsaw Pact representatives have been engaged in the Mutual and Balanced Force Reduction talks in Vienna for a decade. But they have made little progress, largely because of the difficulty of verifying the numbers and movements of troops.
The Soviet Union has recently suggested that it might now be willing to allow visits by Western inspectors to East-bloc countries to verify the withdrawal of troops. But this does not answer the key US concern, which is the number of Warsaw Pact troops now in Europe. The West claims there are about 160,000 more East-bloc troops than acknowledged by the Warsaw Pact. Until this is resolved, most observers believe, progress in this arms control forum is unlikely.