I think people who are confused about their sexuality can sometimes feel a lot of pressure to pigeon-hole themselves in a specific category. People see your sex as male or female (ignoring intersex and gender identity disorder); and sexuality as straight, gay, or bi, ignoring all the 'in between' space on, say, the Kinsey Scale. And things like preferences and other sexual interests don't get classified this way at all, although they might be more meaningful for some than sexual orientation (like fetishes).

I guess the point I'm trying to make is that, despite whatever label there is attached to it, you are what you are. Accepting yourself and your feelings, no matter what their implications might be or how ambiguous they are at the time, is often a more productive focus than trying to figure out if you're straight or bi. You may find out that you do like both sexes a lot, or that you may have some minor feelings for women but still prefer men sexually. Either way is fine, so long as it's what you really feel.

I can understand that you might want more answers and to understand yourself better though, and this isn't wrong either. I don't think I can fully answer this question for you but for me, personally, having sex to figure out my sexuality would have been wrong. Sex is a very intimate act and is a major bonding experience for me. I can't just engage in casual sex, and experience no repercussions. Thus, to have sex with a random woman (a.) might not have worked well as a test because there was no emotional connection and (b.) would have actually been an act of self-harm. And if you enter into a relationship with a woman under false pretenses, and find out you're not really interested in her sexually after committing the act, you could very well be causing her emotional damage. In these circumstances, I think it's often better to just leave yourself open to possibility, and take things as they come if you start developing real feelings for someone. Don't force those feelings on someone they might not be right for - gender is only one aspect of compatibility.

I myself am bisexual, and when I was younger I did have some major feelings of confusion regarding the issue. However, a lot of it was because I did not want to accept the attractions I was experiencing. I would not have called myself prejudiced, but looking back on it, I think I was in a way. A lot of my confusion cleared up when I discovered the true cause of it wasn't in my sexuality, but the way I perceived that sexuality. Perhaps a similar form of introspection on the subject could be helpful for you?

Sorry, I know I'm all over the map here. It's just a complicated subject, and I don't think anyone can really answer it for you. 'Cause that answer lies within you.
If you want a short answer to your question:
If you don't normally and/or easily engage in casual sex acts, I would definitely not advise using sex at this point to try to determine your sexuality. The information you get from this might just confuse you more anyways. (Say, if you didn't like sex with her but might like it with another girl.) So it can't tell you anything for sure and is risky.