By definition, a lesbian is someone who is predominantly attracted to women. Bisexuality is about feeling emotional and sexual attraction to both genders. These categories are just guidelines and never take into account the full complexities of each person’s sexual orientation.

I think that people live out their sexual orientation in different ways, which means that there are no definite characteristics of “what a lesbian is”. You should probably ask yourself why you need to make this distinction in the first place. Bisexuality does not mean that a person is promiscuous, or incapable of long-term loving relationships with one or the other gender. A woman may have dated men for a long time and then be in a relationship with a woman and never date men again, yet still identify as bisexual. A woman might date men and women simultaneously, and still identify as a lesbian, if this is her preference. There is no way to tell, nor any good reason to. The only way to know is to ask.