FROM EL SALVADOR'S rebels come hedged signs of interest in "humanizing" the war to reduce the civilian toll and in exploring both a cease-fire and eventual participation in government- run elections. Do these signs indicate that the Salvadoran left is finally bowing to the people's evident desire for peace and to the advantages in continuity of aid and purpose that President Duarte's election and President Reagan's reelection have conferred upon the government? Or are they meant simply to distract the government and its American patrons, while the guerrillas use time thus bought to prepare for harsher battle later?

The deep suspicions and still-live conflicts of the Salvadoran civil war rule out a conclusive answer now. That such questions can be asked at all, however, is evidence that the peace process begun last month at La Palma is promoting new modes of thought on both sides. From the expectation of endless battle -- a condition that plays into the hands of extremists -- the struggle in El Salvador has moved to an in-between state in which proposals for accommodation are being offered and the civilians who are offering them are necessarily coming more to the fore. Just the other day the two sides saw fit to take part in, of all things, a public debate in Los Angeles, in which each sought to put its best foot forward for an American gallery. With the terms of a second meeting in El Salvador still being discussed, it is obvious that the sides are only at the beginning of setting out a negotiating agenda. That exercise, however, is drawing out a familiar set of political dynamics: within each camp the military wing suspects the political wing may be soft. This is the virtue of opening negotiations. It does not ensure that the faction more inclined to test negotiations will hold its own, but it sharpens the issue. From the public evidence, neither President Duarte nor his counterpart on the left, his former running mate Guillermo Ungo, has yet convinced the other of his seriousness. This they must do. Despite their falling out, they still have more in common with each other than either has with the hard-right and hard-left allies they subsequently found.

President Reagan's firmness in support of the Salvadoran government unquestionably warmed the climate in which peace talks began at La Palma. Whether that climate can be sustained if things get out of hand next door in Nicaragua, however, is very doubtful. And President Duarte still needs steady American help in drying up the death squads and in advancing the sort of economic policies that give him a political payoff. Otherwise the war will go on destroying El Salvador.