Firstly, be sure they have a relevant qualification, or relevant experience. They should be open with you about this.
You also need to feel comfortable with them, and to feel that you can trust them. They might well need to challenge some of the things that you say or do - that's a good thing, and you'll only be able to let them do that if you have a good rapport with them. This doesn't just apply to alcohol treatment: GP's who are warm, friendly and reassuring are more effective than those who aren't.
The people best equipped to help other people are not only warm, accepting and empathic: they're curious. They can really get alongside you and solve problems with you. I think people motivated by this genuine interest in others are better for you than people who are motivated by a desire to 'cure' you, or rescue you. This latter group are often more interested in their own reputation and their own heroics than they are in you.
Remember, a skilled helping professional needs the same ingredients as a top athlete: raw talent, training, experience and temperament.