we used to know a week or so before a test...I would use a product called READY CLEAN...it's a drink you take the morning of the piss test...2 hours after drinking it,and for 5 hours,you will piss clean...but you have to follow the instructions and not smoke for 48 hour before the test and drink shit loads of water
I used this for years, I think I took a total of 7 test...passed everytime at a strict drug clinic ( concentra )...and I smoke everyday ( except 2 days before the test of course)
The only people I have ever heard of this not working for are those who smoked in the 48 hours before test....Good luck