How would I know if I have the right therapist?

I would look up therapists who speak to what you are going through on their website or directory listing and seem to align with your values (some people want a therapist of the similar faith, gender, or cultural background).  I'm not saying that they should have these traits but for some people that's important and helps them to feel more comfortable.  When you find one that seems to fit on their listing or website, try the free consultation that most therapists offer.  Ask them about their experience in treating your problems and how they might be able to help.  I would try them for a few sessions and see how well y'all mesh.  Ultimately if you don't feel like you're getting anything out of it then I would suggest either letting them know if you need something different or try a different therapist.  Your therapist needs to fit like a comfy pair of shoes!  Sometimes that does take a few sessions though as the first stage always involves getting to know each other and building rapport.