Do you mean well-known drugs or new drugs? Tests have been developed for well-known drugs, and at big parties there are often harm-reduction charities offering free or cheap drug testing.
I mean established ones. I'm aware of the narcotic safety tests, but have no idea how one might check the quality of a psychedelic like LSD before taking it.
Reagent tests are the standard for that. They're basically a bunch of acids that change colors depending on what they're mixed with. You test some samples with a bunch of them, and the way each one changes colors tells you what is or isn't in the samples. Ehrlich reagent is good for checking for the presence of LSD, others are good for ruling out adulterants. Fent strips for example are used to check for presence of fentanyl. You can get all of the above from places like DanceSafe.