The way to tell if someone is using cocaine is easy. Just leave a small pile of flour on your coffee table and walk out the room. If it is gone when you come back, then that will answer your question. All jokes aside, one way you can tell if someone is using cocaine is that they might have a runny nose all of the time or sniff a lot. One of the top reasons is needing money all the time. Their eyes may be bloodshot red all the time.