How do I know if a woman is bisexual?

You cannot just tell a person's sexual orientation by looking at them.  It usually is considered rude to just come out and ask them.

Well judging by appearances, you can never tell if she is bi or not. Bi people, just like with all people, come in different shapes and sizes and looks. There are really only three ways to know.

The first is that you've seen her with a girlfriend as well as a boyfriend (maybe not at the same time but you know that she does date both genders). Although this may be kind of difficult to see if she prefers one gender over another (meaning that you see her with only a certain gender most of the time) so this isn't a reliable way to know.

Another way is that you ask her once you are close to her. It's not polite to forwardly ask someone about their sexual orientation if you don't know them well enough. That's why if you want to know, get close to her and then ask her.

The third way is that she was the one who told you by her own free will. Some people don't have a problem telling others about their sexual orientations so you may find out by her telling you directly. I would say that finding a person who is so straightforward like this is rare but they do exist.

She likes both men and women.