Only its high wrought-iron fence and frosted windows distinguish the Soviet Office of Fisheries from the other two-story family homes in the quiet residential neighborhood at 16th and Decatur streets NW.

Yevgeniy N. Chaplin, a first secretary at the Soviet Embassy, is assistant to the embassy's fisheries attache who works out ofthe Decatur Street office and lives on the second floor with his wife. Chaplin said his office is the liaison with Washington for the only ongoing joint U.S.-U.S.S.R. fishing venture, now taking place off the coast of Seattle.

Most of his neighbors "are women and very nice," Chaplin said, and none has accused him of being a KGB offficer, a thought that makes him chuckle. "We only have one antenna here and that is for cable TV. And we have one small radio set."

However, the FBI says it believes Chaplin is a KGB officer.

Like other countries, the Soviet Union assigns many of its spies to jobs in its diplomatic or trade missions as "covers" for their true roles. In trying to identify intelligence officers among employes of Soviet missions here, the FBI analyzes their contacts, movements, position in the mission, and the kind of information they are interested in gathering.

Stanislav Levchenko, a former KGB officer who defected to the U.S. in 1979, offered one clue to identifying Soviet intelligence officers: "The most hardworking people; the people who work the longest hours, who check in on weekends, national holidays, most of them are KGB or GRU."

"They have much more freedom than non-KGB, more than diplomats, not just because of expense accounts. They have to work in the streets; they don't just sit in the residence. You have to talk to people, you have to meet them. You have to go to cocktail parties, receptions."

The greatest achievement for a foreign spy of course is to recruit an American to pass on inside information. U.S. intelligence officials say there is always a hidden agenda when any Soviet journalist, academic, embassy attache or trade representative approaches an American. "No contact can be considered innocuous," said one FBI agent.

James E. Nolan, former FBI deputy assistant director, says "there aren't Americans waiting to be plucked out of trees" to be Soviet recruits. But the Soviets will show utmost patience and sophistication in courting an American, Nolan said. "They're friendly, they criticize the Soviet Union sometimes themselves. They take the approach that there are imperfections in both systems and that with understanding between agreeable people things can be solved. . . . They may ask for a company newsletter, the next time for something proprietary to the company which is not classified but not available to the public."

Some attempt to recruit Americans "is happening every day," said Britt Snyder, director for counterintelligence and security policy in the Department of Defense. "Fortunately, many tell us about it. But how many do we not know about? That's a constant concern. There is no reason to believe that this is a huge problem, but one person can cause you an enormous amount of damage."

Naturally the Soviets have the same fear about American espionage and the U.S. Embassy in Moscow is often called "a nest of spies" by Soviet officials, one U.S. diplomat said. And some observers say that while Americans should be cautious of Soviet contacts, the FBI exaggerates the problem. Dmitri Simes, a Soviet emigre who teaches at the Johns Hopkins School for Advanced International Studies, says it is "silly to tell people not to deal with the Soviets. It is very important for diplomats to remain in communication. The FBI would want to cut off most of that."