As President Reagan's ringing words of support for Caribbean Basin development faded away, business groups, lobbyists, commodity traders and foreign governments settled down to the scut work of sorting out the economic winners and losers.

But as of late yesterday, many of them were still examining the fine print and asking questions about the complicated amalgam of trade concessions and investment incentives.

The Dominican Republic looks to gain income by the lifting of duties on its sugar exports to the United States.

But how will that sit with Brazil and Australia, which currently lead in sugar exports to the United States but will not get the same trade concessions as the Caribbean sugar producers?

U.S. companies that manufacture semiconductors and toys in the Far East have told officials they might move their operations to the Caribbean if the tax and trade concessions turn out to be sufficiently generous.

But how would American allies in Asia, beset with their own economic problems, react to the flight of local light industry? It is possible they will respond with new incentives and enticements of their own.

"We haven't sent the package to Congress, so nobody really has the specifics," said one senior official. "It might be that there will be no losers at all. It could involve billions of dollars, or it could have no impact. My guess is it will be somewhere in between."

The U.S. Chamber of Commerce's Michael A. Samuels said he was "impressed with the comprehensiveness of the package," but added that the chamber needed more time to study the practical impact of the proposed free-trade area.

One question raised by Samuels was whether Japanese business might exploit the proposal by quickly moving industries to Central American countries to take advantage of duty-free exports to the United States.

Excluding the area's $2.7 billion of petroleum shipments to this country (mainly from Caribbean refineries), about 87 percent of the imports from the region already are duty-free.

Officials said yesterday that the plan may be less advantageous for existing industries and commodity sectors than for new ones that could develop as a result of business confidence inspired by the 12-year duration proposed by Reagan.

Under discussion is a five-year extension of the domestic investment tax credit for up to 10 percent of a company's assets invested in the Caribbean states.

Officials acknowledge, however, that the proposal raises some complicated trade issues whose resolution could take months of technical negotiations.

One such issue is the impact of the plan on foreign and domestic sugar growers--always a matter of extreme political sensitivity.

Sugar is the leading export for a number of Caribbean countries, and the United States is the major market.

However, the amount of sugar that the United States could buy from those countries even under a new preferential arrangement is limited by the requirement in the 1981 farm bill to support the U.S. cane and beet sugar industry through a complex system of duties and fees.

In addition, U.S. sugar imports are limited by the fact that consumption in this country has been steadily declining. Sugar imports are expected to drop from five million tons in 1981 to just over three million tons in 1982.

Most countries in the Caribbean already export sugar here duty-free under the generalized system of preferences (GSP). The exceptions are the Dominican Republic, Guatemala and Panama. Under Reagan's proposal, the duty-free concession would be extended to these countries, but their imports would be limited by quota to an estimated 1.15 million tons a year, an increase from present volumes.

Although most of the gain is expected to go to sugar exporters, some is expected to go to U.S. traders, who will be able to purchase Caribbean sugar at slightly below world prices and mark it up to the U.S. support price.

Department of Agriculture officials predicted yesterday that these new arrangements would cut into sugar exports of such countries as Brazil, Australia and Argentina.

They also speculated that the plan could indirectly act to shore up sugar prices in the United States, because the U.S. government may have to raise the fees or surcharge on imported sugar to protect U.S. sugar growers from being swamped by Caribbean sugar. Such protection is required in the 1981 farm bill.