How can I tell for sure I'm gay?

Gay people have feelings only for their own gender. They want to date them, be in relationships with them, marry them, etc. If this isn't how you feel, you're not gay. You may be "bi-curious," which is an interest in the same sex, or "bisexual," which is physical and emotional attraction for both sexes.

Most young women these days have an appreciation of the female form, so your description does not surprise me. If I had to guess, I'd say you were straight.

It sounds like you might be bicurious, or maybe bisexual. There's a lot more options as far as sexual orientation goes besides straight and gay. It's more of a spectrum than a binary. Since you have stronger feelings for guys than girls, you're probably not a lesbian, but you may very well be bi.

I couldn't tell you for sure how you should identify yourself sexually, because it's something that each person has to decide, but my feelings towards guys and girls are a lot like yours and I self-identify as heteroflexible. Basically, this means that while I don't actively pursue girls and am mostly interested in guys, I would be open to a relationship with a girl. Bicurious would be another word for this. It's really something that you have to figure out for yourself, though, and I wish you luck!