I'm not really sure I even want to say this, but in the real world, few employers outside of governments can even afford to test employees for THC. But they
all lie and say they can.
When I fact-checked an article a friend of mine wrote on drug testing, we discovered that by far the most popular "drug test" in use was actually the
http://alcoholism.about.com/od/work/a/etg.htm">EtG test, which only tests for alcohol and cannot distinguish between whiskey drank that morning and a hand-sanitizer used in a public bathroom the night before. Lowe's and Wal Mart both attempt to sell this as a "drug test" to potential employees.
What "drug testing" is really about is a) intimidating actual drug users from continuing the hiring process; b) weeding out the alcoholics, who are by far the most numerous and problematic drug-using employees; c) instilling in the mind of the employee the falsehood that an employee's actions outside of the workplace are knowable by the employer through an inexpensive test, which is total bullshit.
The testing process, and interrogative tactics, are actually used to elicit confessions from stupid employees, which is the only truly reliable and cheap way to catch potsmokers and coerce them into quitting, so unemployment doesn't have to be paid. Just say "no," over and over, and
never admit to such activity to fellow employees.
Even if a real test is used, it's almost certainly too expensive to test everyone at your workplace, so a random (or suspected) sample of employees is taken instead. An alarming percentage of real tests still turn out to be false negatives, and a much smaller percentage are false positives, which are a potential liability to an employer. The testing companies are corrupt, too. It's much easier to send back a printout that says "negative" than it does to actually test a sample sent from god-knows-where.
My friend's research also suggested that employers who
suspect employees of being potsmokers are much more likely to take a urine sample, not test it, and then later confront the potsmokers with imaginary results (again, back to the confession). So stick to your guns and attempt to force your employer to blow that $1000 on an actual test, even if you know you're guilty. At worst, you'll lose your job working for assholes; more likely they'll back right off and never mess with you again.