The Reagan administration is drawing up tough new verification provisions, including on-site inspections, to attach to any agreement that may be reached in current nuclear arms control negotiations between the United States and the Soviet Union.

An interagency working group is drafting final recommendations for White House national security affairs adviser William P. Clark so they can be presented to the Soviets this fall at the negotiations in Geneva on intermediate-range nuclear weapons in Europe and strategic nuclear weapons targeted on the superpowers.

One type of verification proposal under study for the intermediate nuclear force (INF) talks, according to an informed source, would give the United States the right to inspect the sites of Soviet SS20 missiles targeted on western Europe to be sure that the Soviets are adhering to whatever restrictions are in the agreement.

The Soviets would be permitted to cover equipment they considered classified, but "they couldn't hide an object as big as a missile," the source said.

To verify compliance with any agreement to restrict long-range nuclear weapons in the strategic arms reduction talks (START), the source said, both sides could monitor the production of giant intercontinental ballistic missiles where they are assembled. This would eliminate the need of costly technology to count mobile missile systems, according to the source.

The Reagan administration also intends to propose that such verification provisions be made as a separate annex to any nuclear arms control agreement, taking them out of the main negotiations on numbers of weapons, sources said.

"In the past, American negotiators made substantive concessions on weapons to get verification provisions," an administration official said. "We believe verification ought to be neutral, and not require us to concede anything."

Some administration officials said they fear that the verification proposals, if ultimately approved for submission to the Soviets, could become a new obstacle to reaching either an INF or START agreement soon. One official involved in the process characterized the on-site inspection provisions being studied as "mind-boggling."

The Soviets have rejected a Reagan administration proposal to add an on-site inspection provision to the 1974 threshold nuclear test ban treaty that limits each side to underground explosions of 150 kilotons or less. The treaty has not been ratified by the United States, but both sides are observing it.

One Pentagon official said recently that the Joint Chiefs of Staff has "been skeptical about verification measures because they didn't want to accept Russian military men at or near our missile sites."

As a result, he said, some of the inspection provisions "are being reworked to the Joint Chiefs' satisfaction."

According to one source involved in the process, the interagency working group "has been trying to come up with some innovative approaches" to on-site inspection and the ability of each side to monitor the other's missile testing through electronic transmissions called "telemetry."

By monitoring the telemetry of a Soviet missile test, U.S. intelligence analysts can estimate the dimensions of Soviet missiles and the size, number and types of warheads.

President Reagan's conservative critics in Congress have complained that the Soviets are hiding their telemetry by putting it in code. This would violate the SALT II treaty, which also has not been ratified by the United States but is being generally observed by both sides.

One source said that the interagency working group is considering a proposal to prohibit encoding of radio signals from missile tests.

Reagan and many of his top aides based their past criticisms of the SALT II treaty, which was negotiated by the Carter administration, on what they considered flawed verification procedures.

"We are going to do it differently from the past," one Reagan administration official said, "and replace the vague language with specifics."