Kelly

The only surefire way is to ask her. You can't always tell a lesbian by the way she looks. Look at Portia de Rossi and I can't tell you how many housewives in the Midwest I've seen that you'd swear were gay.

My question to you is why do you want to know? Are you interested in her? If not, then her sexuality shouldn't make a difference and you should let her come out to you in her own time. That is if she is even gay.