How can you tell if a guy is hitting on you?

You can't. If you legitimately can't tell you're dealing with the type that are too cowardly to be direct and if you call them out on it they'll get a butthurt and say "Don't flatter yourself". Instead they'll try to weasel into your social circle and follow you around like they have "dibs" and will get pissed off if you end up thinking they just want to be friend when you inevitably get into a relationship with someone else.