It took me a few tries before I landed with a therapist that really worked for me. Back then I didn't know as much about therapy and so stuck with a few that were less effective because it didn't occur to me that I could change.
I think you have two options-
A- stick it out for a few session, and then see how you feel about her
B- interview a few therapists in your area, and choose the one you like best
I wouldn't stay with an average therapist if there were possibly better options out there. therapy is MOST effective when you and the therapist click, so if there's a possibility that you'll like another therapist better, why not give yourself a chance to find out?
Sometimes it can take a few sessions to really get to know a therapist and see if they will work for you, so giving her a few session is a valid choice. I think its always a choice actually- say you look at a few therapists, choose one you think you like best, and then after a few sessions realize that something is off, it would still be ok to cancel and try another therapist.
You asked how do you know, I'm not sure. I knew right away that my current therapist was a good fit, i was very scared when I went to see him and had some space issues. He picked up on it right away, when I asked him to scoot back (even though he wasn't by any means too close for most people) he scooted back as far as he could and assured me it was safe. That was exactly the right response. The therapist I tried before him, sat too close for me, and when I asked her to move, instead wanted to analyze my space issues while remaining too close. She didn't even last the whole hour
While her methods probably would have worked for others, for me it was the wrong approach. I was extremely uncomfortable, bordering on panic, and her response I felt was not helpful and not at a level I was ready to work at. I'm sure its different for everyone. I just knew I felt something click when my therapist said exactly the right thing, and I knew he would be the one for me.