Knowing when you can kiss a chick is something you can only figure out with experience. Usually after you fail to do it, it hits you that you missed your chance, and hopefully you won't miss it the next time you get the "kiss me now" look from a girl. I blew this one a few times until i recognized the look.
Generally, the "kiss me now" look is characterized by a long pause in conversation along with some heavy eye contact (with dilated pupils, or as RJ puts it the "Doggy dinner bowl" look). You just gotta go for it, because most times if you don't, the girl is gonna think that you're either a chump (or gay) or that you don't find her attractive, both of which are really bad, and can kill whatever you have with the girl.
Be like nike and just DO IT.