Barringer Technologies Inc. introduced its DrugAlert test three weeks ago, and its 800 number has been hopping ever since. The $19.95 mail-order kit contains a treated swab that parents can use to wipe down surfaces their kids regularly come into contact with-a desk, a textbook, the inside of a car -to pick up the microscopic drug traces that users leave. Parents send the swab back to the company, which tests it for six different drugs, from marijuana to heroin, and returns results by mail or over the phone in about 10 days. Barringer president Kenneth Wood says DrugAlert gives parents “some early-warning capability… to identify a problem and get real professional help before it’s too late.”

Which could very well be the right prescription. But parents should be wary of using the test to play Big Brother (or Big Mother) around the house. For one thing, a positive test result isn’t in itself evidence of guilt. “It doesn’t mean the person is using drugs,” says Edward Cone, a research chemist for the National Institute on Drug Abuse. “They could have been around anyone who was using drugs and picked up residues in a variety of ways.” Simply borrowing a comb or brush could do the trick. And, though Wood says cops have used similar tests for years without problems, bogus results are always possible. He emphasizes that DrugAlert is “just one piece of information for the parent to use.”

The big picture is important, agree specialists on adolescents. “It should only be done for cause,” says Steven Jaffe, associate professor of child and adolescent psychiatry at Emory University. “If a child is functioning and doing well, you don’t want parents running around testing him and making him feel paranoid.” Signs that might justify a test include running away, shoplifting or declining grades. And if you decide a test is in order, think twice before you tiptoe into your son’s room while he’s out. “I would do it in the open, not sneak upon them,” says Jaffe. Doug Hall, of the Parents Resource Institute for Drug Education in Atlanta, agrees: “The first thing the parent ought to do if they have any suspicion is to sit down with the child.”

Teens hanging out at a mall on New York’s Long Island last week applauded that. “Even though it’s your parents’ house, you still need your privacy,” said Phil Candella, 17, a junior at Chaminade High School in Mineola. “I’d rather they confront me than do something behind my back. That’s a lack of trust.” Still, many said parents had a right to know what kids were up to. Said Chaminade’s Matt Buletti, 15: “The kids that are doing it may not want their parents to find out, but it would be better for them in the end.” For parents, it’s like Ronald Reagan said: “Trust, but verify.”