While getting a colonoscopy, can doctors tell if you've been smoking marijuana or drinking alcohol a couple days prior?

No: A colonoscopy will not disclose that - but you should be honest with the anesthesiologist - certain drugs could alter your anesthetic

No: To find that out they would have to do blood and or urine test.Neither would indicate alcohol use after a few days. but the weed would show in specific testing.In many states they have to have you sign a permit to these for these before they do it.