"PEOPLE," warns Louis J. Rose, "should be aware that there isn't much privacy left in the United States. They have to know that--if they want to protect themselves."

Rose, an investigative reporter for The St. Louis Post-Dispatch, hastily adds that while privacy ("a vanishing virtue") is being diminished by institutional forces in this country, it's a double-edged sword: "If certain records were not public, for example, it would be impossible for us to check whether public officials were acting to line their own pockets or to benefit the public."

On the other hand, we should be aware that there's a tremendous pile of information about us building up out there in our computerized society. Rose emphasizes, however, that the information is available, for whatever reason we might want to turn private detective:

You're applying for an important new job. The person interviewing you has a complete work history on you, checked with your references and work associates. Although you know only the interviewer's name, you may be able to track down enough information to give you an edge on other candidates for the job.

Probate of a relative's estate is delayed, pending location of a missing relative named in the will along with you. You want to find the missing heir.

You're thinking of putting a parent in a nursing home but want to be sure of the home's reputation for quality care.

You're thinking of investing in a venture or going into business with another person, but have misgivings about him. You can check up on his performance in past business ventures and standing in the community.

You're about to become involved in a divorce and want to check out your spouse's true assets.

Or, there may be property that is an eyesore in your neighborhood. "Say it's a dilapidated building--a hazard to children--and there's an absentee landlord. It's quite easy to go into the records and find out who owns it and complain about it."

Rose cites the case of a man who complained to his local government about just such a property. "My friend showed his dad how to look up the records. When he did that, he discovered that the property was owned by a man known to be a friend of the mayor.

"His dad went to a city council meeting and said, 'I don't care if this thing is owned by John Doe, dearest friend of our mayor. I want it cleaned up.' In two weeks it was cleaned up."

On another occasion, Rose says, he was asked for help by a friend whose husband had died. "She was concerned because a man she was seeing was always pressing her to marry him. He repeatedly made the point that he had never been married. He was 35."

The woman checked the local property records while Rose checked the state's motor vehicles department. "His car registration records came back with two names, his and a woman's," says Rose, "and the property records came back the same--with his name, a woman's name and et ux., meaning and spouse. The guy had been lying to her and she broke up with him."

Rose, who has written a guide, How To Investigate Your Friends and Enemies (Albion Press, 137 pp., $7.95), says the average person can even check out suspicions about how their local government is spending taxpayers' money. The process is comparable to going through your own checkbook to figure out where the money has gone, only you're checking 50 or 100 community checkbooks.

"Say you're concerned with money for snow removal," says Rose, "or with some types of particular purchases. You can check that account and run it down and if you see an awful lot of money going to certain people or to a certain company, you can find out whether those things were bid, whether that company was operated by a city official or friend of a city official."

On a more personal note, Rose, 51, stresses that people should be able to see what records are available on themselves, "to find out what privacy they have left that's vulnerable." Under the Fair Credit Reporting Act, for example, you have the right to go to a credit bureau, see your credit record and "challenge it or to try to amend it." You may not be able to change the record, but if it contains incorrect or unfavorable information, "you'd better put an explanation or challenge into the records on that."

The Freedom of Information Act of 1966 "gives us a chance to find out what files, if any, the federal bureaucracy may be keeping on us or on groups we belong to." And under the 1974 Privacy Act, private citizens have the right to inspect and challenge any information about them in federal files.

A guidebook on the FOI and Privacy acts is available from the FOI Service Center, c/o Reporters Committee for Freedom of the Press, 1125 15th St. NW, Washington, D.C. 20005.

"If it means enough to them," Rose insists, "the average citizen can do a whole lot. They may have a lot of opposition, but they still have a right to that information."

In dealing with public servants, he offers this further tip: A little courtesy goes a long way in gaining access to information.

"That does not mean, however,that we are justified in invading someone else's privacy without the most compelling reason."