They NEVER tell you anything at all! You will wonder FOREVER! Just kidding. The lab will contact you usually and ask why you have marijuana in your system blah blah blah. If you pass the company will contact you and tell you to come in to start. I failed one time and got a call and said omg I got the job! "MR is there a reason you have marijuana in your system at high levels?" If you haven't been contacted within a few days you should be in the clear. They called me literally the day after and told me I failed.
They never call you either way. Unless you popped positive on a narcotic that you might have a prescription for. Then they will just call and ask for the name of your doctor to verify. Usually no news is good news so just figure starting on your start date.