The Soviet Union, freed from the provisions of the SALT II agreement by President Reagan's decision to consider the pact a dead letter, could now test-fire a dozen of its ICBMs from concealed launchers simultaneously without first informing the United States.

That test, which might look much like a real attack to American satellites and radars, could only have been conducted after Moscow informed Washington under a little-publicized SALT II provision requiring advance notification of any test involving multiple intercontinental ballistic missiles or one that sent test warheads outside Soviet borders. The provision was proposed by U.S. negotiators so "neither side would be frightened when they saw more than one missile being launched," according to a former State Department official who was involved in arms control matters.

Since the president has said the United States will no longer be bound by the SALT II strategic arms limitation treaty, "it is difficult for us to argue that [the Soviets] are bound," Assistant Secretary of Defense Richard N. Perle said yesterday. "No one" in the American government, he added, "has been cast to go out and think of diabolical ways to depart from the [treaty's] provisions."

There is no assurance, however, that the Soviets will continue to observe the treaty's provisions -- either those governing the number of permitted weapons on each side, or the many less-publicized sections of SALT II designed to stabilize and regularize the competition between the superpowers.

Roger Molander, a former White House aide who worked on arms control issues in the Nixon, Ford and Carter White Houses, said yesterday that the SALT II rules had the effect of making new modernization of strategic weapons "predictable and increasingly boring." As a result, he said, proposals to build ever more offensive weapons lost much of their appeal. Molander also said that when both sides could see and assess the other's nuclear programs, both felt relatively secure.

That could now change. One provision in SALT II praised by its supporters prohibited deliberate concealment of weapons tests and deployments by both nations. The Reagan administration has charged that the Soviets have violated the treaty provision that prohibits encoding essential data from missile tests, but there are many other important concealment provisions with which they have, to date, complied.

If they now decide to change that policy, the Soviets could begin to cover over silos of deployed missiles, hide the locations of their new generation of SS25 and SS25 mobile ICBMs and severely limit what U.S. spy satellites can see from space, including whether new holes are being dug anywhere in the Soviet Union for new missile launchers.

The Soviets "can blind those KH11 [U.S. photoreconnaissance] satellites anytime someone gives the order," said Molander. "That closed society will have a lid on it" if the Soviets opt for concealment, he added.

"One of the most important losses [from abandonment of the SALT II agreement] would be in the denial of information," according to Jack Mendelson, deputy director of the Arms Control Association and a former Foreign Service officer on the SALT II delegation during the Carter administration and the START (strategic arms reduction talks) delegation under Reagan.

He said it would be impossible to measure the effects if "we let offensive missiles be concealed."

Another SALT II provision prohibits the Soviets from putting more than 10 warheads on their presently deployed, giant SS18 ICBMs, considered by Perle and others to be the most dangerous Soviet missiles. A basic part of the Reagan administration's arms control policy up to now has been to get deep reductions in the SS18 force.

Without SALT II, however, the Soviets could increase the number of warheads carried by the SS18, a step that could be taken fairly quickly since the Soviets established with tests in the late 1970s, according to U.S. intelligence, that the missile could carry up to 14 warheads.

Perle said yesterday he believes the Soviets may already have put 14 warheads on some SS18s. Lacking SALT II, they could openly test that capability and quite quickly add 1,200 new warheads to their force.

The Soviets have also been developing a successor to the SS18, and according to one former U.S. official, they "are on the verge of the first test flights." That missile has probably been developed to carry 10 warheads because of the SALT II limits but, he added, it "shouldn't take more than two years to develop the capability for it to carry far more than that number." Theoretically, a missile the size of the SS18 could carry as many as 20 warheads.

The SALT II rule that limited the number of warheads on land-based missiles to 10 was done "to protect our own missile force," according to a former aide to the Joint Chiefs of Staff on arms control matters.

"There was a linkage [between the size of their forces] and the survivability of our systems," he said, referring to the fact that SALT put an upper limit on the number of accurate ICBM warheads the Soviets could aim at U.S. targets.

The SALT agreements also made use of the U.S.-Soviet Standing Consultative Commission, established in Geneva under the 1972 Antiballistic Missile Treaty, as the forum for raising questions of compliance with the arms agreements.

Administration officials have acknowledged that they will no longer raise any challenge to Soviet behavior at the consultative commission meetings on the grounds that it violated SALT II. Meeting with reporters and editors of The Washington Post yesterday, Perle predicted that the Soviets would commit additional "violations" in the future, but under questioning he acknowledged that the word violations could no longer be applied since Reagan decided to cease respecting SALT II.