I would first do my research on internet and when I pick three names, for example, would ask for personal refferences in my friend circle. I could even make an appointment with the three therapist and personally check how I feel in their company. A good reputation talks itself very convenient though it is a personal choice to whom you pay trust credit and share your intimate issues with.
Marriage counseling is not the usual topic to discuss on friends dinner party and it's not because it is a bad thing but when you mention it people think you have some serious problems and start looking at you with pitty. You don't need that kind of attention. Better ask about it one person in private and double check the provided information :)
There are many ways to go about finding a marriage counselor, but usually the best way is to find a referral from someone you know. This way you can get a better sense of how the counselor operates and see for yourself if the results are what you’re looking for. It helps if you can get more than one referral, since you should evaluate your options after first calling and then having an initial information gathering session. During the first session, the counselor will ask questions about your history and you can get a better idea if you’ll get along them and whether you can trust them.