How do you know you're attracted to someone?

For a long time I have labeled myself as asexual for I had no interest in anyone. However, recently I have begun longing for a relationship. But, I'm not entirely sure if I'm more attracted to girls or guys.

If I see a good-looking girl, I don't get very nervous (probably because she is unlikely going to realise I'm interested in her), but I won't be able to stop looking at her and feel a bit fuzzy inside

When I see a good-looking guy, I will get really nervous and try to not pay any attention to him.

I can imagine myself being happier and more comfortable with a girl, but then being with a boy doesn't feel wrong either.


I'm still so confused about my feelings, so I'm wondering how you feel when you are attracted to someone? How do you know you'd like to be in a relationship with them?
Also, if you aren't bisexual, how do you feel about the gender you aren't attracted to? How do you feel when you are around them?