Here are a few articles from Psych Central with some advice on your question:

Finding a Therapist

Questions to ask your Therapist

Interview Your Therapist

I think one of the most important things is whether you can connect with your therapist. You would probably know this within the first 3 sessions. Also, if you need a T with a speciality, for example in trauma or addiction, you might need to ask right away if they do this sort of work when you phone them the first time. Theoretical orientation can also be very important. Good luck!