Just stick to girls on their own, or maybe with 1 other girl until you get more confident. Work your way up.
If shes with a guy, normally you can tell if they're together by the body language. If you're not sure then you can always ask her... "hey, is this your boyfriend?". Worst case scenario is she says yes. If a guy ever freaked out (which is highly unlikely if you're polite) he would be making an absolute fool of himself in front of her anyway.
I also bail from targets occasionally from fear of them being too young. I follow my instincts on that one. Perhaps you can use an opener that might require them to reveal their age?
Example: "Let me guess, you're 19 and you were born in February?"
Personally, I wouldn't in case they say "actually I'm 15".
