If you look up testing laboratories on the net you'll find one. Some will send you a kit for you to place a sample in and send off. I've heard these are not as reliable as you need an intact follicle.
The test I believe runs about $300.00 and will show if there's been a pattern of use over the last 6mos to a year.
There are three basic reasons to use this test
1. To prove you have never used.
2. To prove there is no long term pattern of use
3. to disprove other false positive tests.
One should only use this test if they are certain of the result as it can hurt you in the long run if it shows long term use.
Peace and Namaste
Hppy