They normally check hair follicles which can show what you've put in your system for the last six months or longer. And if one shaves their hair, body hair included, it's normally taken as an automatic failure to comply. It is pretty tough, almost impossible, to beat a follicle test. When all else fails it's a good policy to use honesty. Good luck and God bless.
I am not a dr. My statements are based on years of experience and related education. Consult with the professional of your choice regarding matters of concern.