Ask someone you can trust to recommend a therapist that they have been to. It is too important to just choose someone in the yellow pages. Ask a counsellor at a school, university or clinic to recommend someone. You could ask your doctor or insurance company for recommendations. Look them up on the internet and check out their website and references.
One way to know whether a marriage counselor is reliable or not would be to hear the stories and testimonies of the people who have been counseled, and to see whether or not their lives have changed for the better as a result of the counseling they have received. If you do not have access to such information, you would need to do some research of your own and check out the qualifications of the counselor and how long they have been providing marriage counseling. Probably the best way to find a reliable marriage counselor is through a referral from somebody you trust such as your pastor or your doctor.