I'm with everyone else on the surprise visit/trial period (if you can swing it). My gelding actually does fall asleep while tacking up and he's really lazy and behind the leg a lot of the time. Just a note on "testing". You can't just screen for drugs so you'd have to know what drug you think they are on and then ask specifically for that drug to be tested so realistically there is no way to test.
I think another thing you want to consider, even if it turns out he's not drugged is whether you want a horse that is this dead and lazy. I will tell you it is a real pain to work with a dead horse and though my guy has been a real confidence builder I'm ready to ride something that doesn't make me work for every stride!!