I’ve already covered that in an earlier post here when I said drug tests should be considered for welfare recipients. By far the vast majority of marijuana users are self supporting members of society contrary to what you may think.
Unfortunately “drug tests” are not useful markers of recent drug use for some recreational drugs. It would be nice if they were, but other, less straightforward measures may be needed in some cases. Proof may be difficult in some cases but the principles of requiring responsibility for drug use and denying drug use to those who can be reasonably proven to be irresponsible, should be the goals.