Reliable and accurate drugs and alcohol tests have been around for a while and are in use by the Police, Prisons and Health Services. Breathe test meters, saliva tests (accurately reflecting blood alcohol or drug levels) and multi-drug urine tests are cheap and easy to use - although urine testing for alcohol seem somewhat irrelevant as urine does not reflect impairment.
So self testing for alcohol is cheap, practical and reliable.
Nobody has specified what "drugs" are or what the cutoff levels are - so no help there (and may not be told when we are being naughty for some time). Most drug tests conform to the SAMSHA standard, which aims to indicate levels in the body consistent with "recent" intoxication.