There are test that can pick up if you have used psilocybin but they are WAYYY too expensive for police or Probation officers to use. Your in the clear. Same goes for LSD and most designer drugs such...
It's mostly algebra, some statistics, and basic arithmetic (with fractions, etc.). It's only slightly harder than the SAT, largely because they try to use your intuition to trick you.
Urine, saliva, and some blood test kits for home/work use Gas-Chromography will test hair, blood, urine, saliva. There are reaction agents that change color depending on if they detect a drug or not....