Am I that good, or is it a game?

Well you are "The shit" so does that answer your question? Just kidding. Well lets put it this way, girls like to flirt just as much as guys. If you are cute and they feel safe around you I wouldn't doubt if you said 100% flirt back with you. If you want to know which ones are serious and which one aren't you should have slightly more serious conversations with less joking. If you are interested in one in particular get her alone and have a semi serious conversation with her. Don't make it too serious. Be careful who you pay special attention to because it may not be the girl that is crushing on you and if you pay extra attention to one or the other you could get yourself in trouble. You work with allot of women and it can be very good or very bad for a man depending on how you handle things. Good luck buddy.

Depends on what you mean by "serious". You can probably manage an FWB with all of them at some point if you're cool, discreet, and straightforward. For a serious relationship, you're going to have to kill the flirting altogether, though it might be too late for that.