Most pre-employment drug screens are for THC, Cocaine, Methamphetamine, PCP, Heroin, etc. They are a simple urine sample that are usually tested in a presumptive kit. Kind of like a pregnancy test kit. Or, they send it to the lab to test for the aforementioned substances.
If they do a blood test, they might look at your test levels, which could bring up a red flag. They may question you about it, but I doubt it would go any further due to the costs.
I've known plenty who have tested in a law enforcement and firefighting setting while using PH's and AAS's, and had no problem.