The success of President Reagan's Latin American trip will be measured by whether and how it leads to follow-up action. In some cases this means proceeding along lines underscored during the journey - pushing the Caribbean Basin Initiative, for example, and improving trade channels with Brazil and Colombia. In some cases Mr. Reagan would be better advised not to pursue prospects he raised - such as seeking military aid for Guatemala.
Making the most effective choices will depend on how administration attitudes are reflected in several positive things Mr. Reagan said:
* ''I learned a lot.'' The beginning of wisdom.
* ''One of the greatest mistakes in the world'' is to lump countries together in Latin America. The President can help his countrymen get away from doing so by responding himself to the individual economic, social, and political characteristics of nations. Thus he would also refrain from homogenizing them in an East-West power context.
* When a left-wing national assemblyman in Costa Rica interrupted a Reagan speech to attack ''the militarization of Central America,'' the President rejoined that the critic could speak out in a democracy but not in a communist land. Here was an example of turning a situation to his advantage with rhetorical skill. An effective follow-up would be to bolster democratic forces against both communist and right-wing authoritarianism.
The trip itself was a gesture of support for a long-time democracy like Costa Rica and nations emerging from military rule such as Honduras and Brazil. Since preservation of human rights is fundamental to permitting the growth of democracy, it was good to hear that the administration was nudging its client government in El Salvador. Progress on rights will have to be made if Mr. Reagan is to provide credible certification of it for legislative aid purposes by the January deadline.
President Rios Montt of Guatemala was evidently spurred by the Reagan trip to make at least vague assurances about future elections. But for Mr. Reagan to suggest that the Guatemalan regime had received a ''bum rap'' on human rights did not so much help that regime as undermine his own sensitivity on the subject. If he goes ahead with a mis-guided effort to get military aid for Guatemala, he will soon get stiff resistance from congressmen concerned about the reports of Indians massacred by the Guatemalan security forces.
Some observers saw the trip in tune with a reduction in East-West confrontational attitudes by the administration. This would be all to the good. Alertness to subversion by Cuba or other Soviet proxies must be maintained. But, as President Kennedy found before him - and Colombia's President Betancur tried to tell him during this trip - the communist menace must not be allowed to loom out of proportion to the social and economic problems that present the greatest threats to stability. And, one might repeat, the greatest opportunities for subversion.