He doesn't look anything like the stereotype of a private detective: a wise guy in cheap clothes and a fedora, constantly smoking and drinking, working out of a seedy office in a seedy part of town.

Nicholas R. Beltrante dresses in conservative, well-tailored suits, drives a Cadillac Eldorado and a Honda Prelude, and works out of a comfortably appointed office above his Alexandria residence.

Floor-to-ceiling bookshelves house a library with something for everyone: volumes of history, philosophy, science, religion, art, fiction -- and especially mysteries. A walking cane with a brass handle shaped like a sparrow (he's a bird fancier) rests in one corner. Plaques from his days as a Metropolitan Police detective, awards, diplomas (he graduated from law school in 1967) and other memorabilia cover another wall. A sign on the desk thanks visitors for not smoking.

And he doesn't keep a bottle of whisky in a desk drawer.

"People," says Beltrante, 57, "have no conception of what it's like to be a private investigator today. They get all their ideas from the TV shows. It's great advertising for us, but I don't care which one you turn on. The guy's been shot at, beat up, run over by cars . . . We just don't run into that on a frequent basis."

Beltrante, who was called in to debug the Democratic National Committee offices in the wake of the Watergate break-in, and who frequently appears on radio and TV talk shows to discuss his profession, recently taught a one-session course at First Class Inc., 1522 Connecticut Ave. NW. (It will be repeated Feb. 13, 7 p.m.-9:30 p.m., $25 fee.) The course, "Making $100,000 a Year as a Private Eye," drew 17 students -- four women and 13 men -- ranging in age from early twenties to 60-plus.

Their occupations ran the gamut: unemployed, truck driver, journalist, engineer, lawyer, retired. Whatever their career status, all had come to get the word from the dean of Washington's private investigators: What is a private investigator, how does one get started, who are the clients, what kind of cases do PIs handle, what's the pay?

Beltrante, who retired from the D.C. police in 1963 after more than 13 years on the force, basically had good news for his audience:

* It's not difficult to enter the field.

* There's business enough for everyone.

* The fly-by-nighters that give the profession a bad name tend to weed themselves out.

* Increasing numbers of qualified people are coming into the business.

Beltrante notes approvingly that more and more women are entering the field, "not only as investigators but as agency owners.

"The complexion of my firm itself has changed dramatically in the last several years. Five years ago the vast majority of my people were male investigators. It has reversed itself now. Out of the total of 12 people I have registered with my firm, including myself, eight are women. Two-thirds of my staff are female investigators. I find them to be more dedicated, more conscientious than the males."

After the class, two of Beltrante's women students approached him about jobs. Both, including one with no experience (but with "a good approach and attitude"), have been hired on a part-time basis.

The second, Kathleen May, offered Beltrante hard evidence of her qualifications. May, 30, who was adopted as an infant, decided as an adult she was going to find her birth mother. It took 15 months, but she succeeded and was reunited with her mother over Christmas, 1983.

The fees Beltrante earns vary, depending on the type of work performed, but clients are likely to pay around $50 per hour for an investigator's time, with minimum retainers ranging from $100 to $1,000 and up.

The retainer fees are paid in advance. "I have no time to collect bad debts," says Beltrante, who charges $1,000 a day and all expenses when he works a case personally. "I've only had to sue one client -- an attorney."

Standard PI work includes civil and criminal cases (often for the defense), personal injury and Workmen's Compensation cases, divorce and other personal matters ("child custody, child molestation, estate cases, kidnaping, missing persons, record searches and surveys and surveillance of all types"), business and government fraud.

In addition to government agencies and private individuals, potential clients include insurance companies, private businesses, law firms and individual lawyers.

In one case cited by Beltrante, the Navy had asked for bids on a multimillion-dollar contract. According to terms of the contract, it had to be a small company: "It was a giant job for a small firm with not more than 10 employes."

The contract was let to a group that later was suspected of being a front for a very large computer organization. "We knew the guy didn't have the capability of doing the work," says Beltrante. "We had to connect him with an organization big enough to do the work. It's amazing how sloppy people get."

In this instance, the suspect was driving a car that was in his name, "financed by the credit union of this large corporation." Beltrante's people went to work and "found that he was in fact on their payroll . . . As a result, all hell's breaking loose on the West Coast right now."

In another case, Beltrante was asked to help the defense in a case in which an 18-year-old woman had been charged with murder. After extensive legwork, Beltrante decided the woman was lying to him, her family and her lawyer.

He confronted the woman with what he had uncovered. "I told her, 'Tell me what happened. Maybe your attorney can plead you to a lesser charge.' She broke down and confessed. We went back to her lawyer. The charge was reduced. She's going to serve a lot of time, but she won't go to the electric chair."