What is the very best way to check if someone is drunk or high while they sleep?

I don't know how: You'd do that. It seems a bit sneaky. Although you could smell someone's breath as they exhale - that wouldn't tell you if they were drunk. Even if their breath smelled of alcohol it could be due to use of a product like a mouth wash w alcohol. Since urine or drug levels (or breathilizers) are usually used to detect drugs/ alcohol - i can not think of a way to do what you are proposing.