Though the State Department is about to make public a mess of intelligence data meant to show how Cuba and Moscow are behind the insurrection in El Salvador, it is the lack of intelligence about what really is happening inside that tiny Central American state that poses the risks and dilemmas for the Reagan administration.

Senior officials, supporters of the administration's clear intention to crack down hard on Cuban-fed insurrection, say privately that the real problem is that the United States doesn't know how severe the threat from leftist guerrillas is to the ruling junta in El Salvador, how strong those guerrilla forces are and how able the government is to cope with the threat.

The United States knows, for example, that hundreds of tons of arms are being shipped covertly into neighboring Nicaragua for smuggling into El Salvador. But the dimensions of the arms supply may be greater than the ability of the guerrillas to use them, and officials say they don't know how much the weapons will help an insurgent force that just saw its largest offensive flop.

Officials here say the United States also knows that the clandestine airlift of arms from Nicaragua to El Salvador is being run by the chief of staff of the Nicaraguan air force. In recent days, sources say, members of the government junta in Nicaragua have privately told Washington that they would try and put a stop to this. But nobody here knows if they can.

While the United States has been able to gather technical intelligence of outside support from captured documents and observation of suspicious air and surface shipments into El Salvador, sources say it has been very hard to gather the kind of political intelligence inside El Salvador that is necessary to evaluate what is going on and who is winning.

It is upon such evaluations, however, that the administration will have to assess what steps it will take to aid El Salvador after its public campaign of finger-pointing at Cuba and the Soviet Union is unveiled. It is understood that U.S. intelligence has recently taken steps to improve its understanding of what is going on inside El Salvador.

The administration has made it clear that economic and military aid, in the form of equipment and money, will be increased to the government there. But what happens after that could be a much bigger step, perhaps involving greater U.S. military involvement, and senior officials indicate that the administration does not really know what it will do.

Some officials say they are aware that the campaign against Cuban support of insurrection in Central American and Africa that has been a loud and steady theme of the new administration carries a risk of whipping up public support for action at a time when it is not clear what action would work. Secretary of State Alexander M. Haig Jr. is said to be keenly aware of the potential dangers of any U.S. military involvement.

If the United States sent in military training teams to help the Salvadoran military shut off the armws flow, officials said the risks are high that some Americans would be killed.

The spector of another Vietnam, whether fair or not, undoubtedly would be raised in this country at some political cost to the new administration. It also could undermine the consensus for greater U.S. defense spending, which the White House and Pentagon want to preserve.

During his confirmation hearings by the Senate Foreign Relations Committee yesterday, the State Department's new counselor, Robert C. McFarlane, told the committee it "may be confident this administration does not intend to make a Vietnam-like commitment of major forces" in El Salvador. The United States could, however, "give them modest assistance to help them in their own defense," he said.

Some sources suggested that if the United States does decide to give El Salvador advisers in a direct effort to shut off the arms flow, they might be Navy teams that would help set up patrols to stop arms smuggling by sea, an area where casualties would probably be less likely.

At the moment, the United States has eight military persons permanently assigned to El Salvador at the U.S. Embassy and in the small military aid group, according to the Pentagon.

Other U.S. servicemen, fewer than two dozen, are there on temporary duty, most training Salvadorans in helicopter maintenance and others helping train pilots and helping the Salvadoran military high command with its operational planning.

While the eventual degree of U.S. involvement in El Salvador is not clear, senior officials say what is clear is that an attempt to halt Cuban military adventurism in Central America and Africa is a central focus of the new administration's foreign policy, and that Haig's strong, personal feelings of animosity toward the regime of Fidel Castro and its policies is at the leading edge of the new and tougher American approach.

The situation in El Salvador, Haig told reporters yesterday, has "progressed to a point where it must be a matter of grave concern to the United States."