The way I prefer to do things is to talk about it (maybe because I'm pretty shy and awkward about it), but I find it's for the best. After my first date with my girlfriend, we kissed. Some time later, I sat down with her and asked "if this made us boyfriend and girlfriend" and we talked about the relationship and agreed it did.
That's how I figure out if I'm a girl's boyfriend. It's the best way to eliminate any confusions about "well, I thought we were just hanging out".