How do you tell if a guy who is very flirty with all girls likes you?
If a guy is very flirtacious with other girls, then you DO NOT want to get involved with him. He sounds like he would just make you jealous and worried about him and question his feelings about you, like you are right now. I would move on and look for someone else who you can clearly read his feelings for you.
Some times a guy that acts flirtacious with other girls is just trying to make you jelous... check to see if he finds times while his is flirting with other girls to look over at you...
Also if they smile at you when they look over at you while flirting with other girls... its a bonus :)