There are no federal regulations for non-DOT tests relating to which drugs to test for and the thresholds to use. You can base your drug tests on your workplace needs and state law. Some states have restrictions on marijuana testing.
It is often best to work with your drug testing lab to determine which drugs are most commonly used and tested for in your area. The Substance Abuse and Mental Health Services Administration (SAMHSA) provides guidelines on drug testing thresholds.