The Fairfax County Police Department, in its efforts to prevent crime and solve cases, has found a dependable ally - senior citizens.

"We find the elderly are more apt to report crime than younger people," said Art A. Lilje, community relations officer of the McLean station. That fact, he said, applies "across the board," whether the senior is a victim of crime of is helping someone else.

As an example of how valuable a single tip can be, Lilje cited a phone call from an older woman who, unable to sleep, had been watching late-night television.

She noted that an unoccupied automobile, parked near a neighbor's home, had its engine running. When its lights went on as it was driven away, she got the tag number and phoned the police. The arrest that followed was that of a professional cat burglar. That arrest closed the books on that burglary as well as 17 others.

Fairfax County has a force of 702 officers, 23 park police and about 300 civilians. Despite those numbers, police coverage of the 399-square-mile county is a demanding job. The most populous political jurisdiction in Virginia, the county population is 580,000 and is projected to rise to at least 606,000 next year.

Because of the the county's size, police find "citizens patrols" also to be a valuably ally. These operate around the clock, using volunteer manpower. The private auto on patrol has a CB radio. If the driver sees suspicious activity, he radios word of it to the "base station" ' a private home - whatever the hour of the day or night. The person in the base station notifies police by phone. Many retired people are active in the patrols that cover Chesterbrook Woods, Stonewall Manor, West Springfield, North Springfield, Friendly Village and Greenbriar.

The patrol unit in Chesterbrook Woods, a neighborhood of homes valued at $150,000 and higher, produced dramatic results. In a three-month period last fall, 150 cases of vandalism and several burglaries were reported in the area. Then last February, retired Rear Adm. Donald Baer, a Chesterbrook Woods resident, worked with Officer Lilje to form the patrol. Since then the area has had fewer than 10 cases of reported vandalism, which all resulted in arrests, plus one burglary and one attempted burglary.

Baer said the unit how has about 230 participants, all members of the Chesterbrook Woods Citizens Association. They follow police advice, acting only as "eyes and ears" for the police.Anyone engaging in a "confrontation situation" would be asked to leave the force, he said. The citizens association paid for the CB radio equipment and for the transferrable signs used to mark patrol cars.

Another anti-crime program available to the community is Operation Identification. Cpt. Thomas L. Shaw, commander of the department's Training Division, strongly urged seniors to participate in the project of engraving Social Security numbers on items of high value or those frequently stolen. The county has made participation easy, he said, by equipping each of its public libraries with six etching machines. These are available to anyone on 48-hour, loan-free basis and without a deposit.

"I have seen thousands of dollars' worth of (recovered) items auctioned off because we couldn't identify the owners," said Shaw.

Fairfax County also offers seniors and others security surveys of homes, apartments and businesses. As in Arlington County, they are free, written and confidential.

The phone number for an appointment is 691-2131. However, questions about a survey should be directed to the community relations officer at the nearest of the county's seven police district stations. The stations, and their addresses and phones, are:

Chantilly, 14214 Coda Place, 691-2426; Franconia, 6121 Franconia Road, 691-3332; Groveton, 6210 North Kings Highway, 91-2116; Mason, 650 Columbia Pike, 691-2244; McLean, 1437 Balls Hill Road, 691-3265; West Springfield, 6140 Rolling Road, 691-2301; Reston, 11415 Isaac Newton Square, 691-2645.