If a girl's looking at you it's on. Just go up and say "do I have something on my face? People have been looking at me all day" with a smile on your face, she'll understand that you're not 100% serious and it's a way to start convo, plus it has added preselection.

Unless you're in my friends situation: walked round Manchester most of the day with a LOT of girls checking him out. Decided that he must had some godly aura of sexiness and decided to start opening girls. Did a daygame sat, went well, and ended up getting her number for her to point out afterwards that he had chocolate sauce down his face. Win.