As others have said, experimenting/putting yourself out there is your best shot at figuring it out as well as reflecting a bit on what your experiences made you feel.
In my case I'm definitely a homoromantic homosexual male. However for some 10 years I agonized over if I'm actually bisexual or not because I never had a sexual encounter with girls even though I knew I could appreciate the female figure.
The clincher moment for me (even though I've had oral sex several times with males) was when I took a shower with my best friend's girlfriend. I had joked we should shower together after swimming because I felt I was gay and she surprisingly agreed. I looked at her and she was very beautiful but my body didn't react in the slightest.
Perhaps thats a poor example but that experience allowed me to completely affirm that I both love and would sleep with men at least 99% of the time. I still allow for the fact that it is possible that there may be a woman out there I'd go for since I've seen some masculine looking ones that I would check out at the very least.