Ask.

i have dated lots of guys, and they just don’t cut it for me. i’ve been in love once, and it was with a girl. i haven’t tried with anyone else of the same sex, but i wonder if i should. how did you realize that you were lesbian?

If you want to, you should do it.  Don’t hold back on what you want to do.  As long as someone makes you happy, that’s all that matters.
I’ve almost always known that I was gay in some form.  Since I was young, the thought of being with another girl fascinated me.  I’ve been in relationships with guys before, too.  I’ve found that I would just rather date girls, but that’s not to say that if I met a guy who made me feel like I need to feel to be in a relationship, I wouldn’t be with him simply because he was a male.  I remember the first girl I ever had a crush on, and that really made a difference in realizing that I just needed to be who I was because my feelings weren’t going to change.
So, be who you are.  If you want to date a girl, then try and go for it.  If it doesn’t work out, then so be it.  Just don’t hold back.