You can ask about membership of The Centre of Applied Pet Ethology (Coape) or some other organization that does some kind of training and quality control before admitting members. You can ask about the therapist's education, whether the person has a college or higher degree in animal behavior. But not all good therapists have degrees. You can ask at the vet surgery. You can ask at a dog school that rejects the choke chain for training dogs and uses treats and/or the clicker instead.
When you contact a therapist, you can present your case briefly and let the therapist start talking. Then...
One thing to watch out for is that the therapist doesn't just have a standard, ready-made answer. A good therapist will ask you lots of questions about your dog and his/her history, how you brought the dog up, when the problem first started, when and where it occurs, and so on. The therapist will want to make a house visit and watch you with the dog. S/he might ask you to do various things with the dog, perhaps even asking you to do something that will make the dog show the growling or snapping behavior. (A good therapist will not ask you to do this in a way that puts you or your dog at risk -- neither physical risk, nor emotional and psychological risk.) A good therapist will take a thorough look at you and your dog as individuals and at your particular relationship. If the therapist already knows the answer before you even tell your story, it's better to look for someone else.
Another very important thing to watch out for is that the therapist doesn't start talking about dominating your dog by any technique that sounds the least bit like intimidation or punishment. Some good therapists will still talk about dominance and ranks and leadership -- but when they start explaining how to change these things, they will not be telling you to intimidate your dog, hit your dog, kick your dog, jerk on the choke chain, nor do anything else that is scary, painful, or intimidating for your dog. A good therapist will know exercises for you that make the dog feel less intimidated and worried rather than more.
The instant a trainer or therapist starts talking about punishing your dog or doing anything that is intimidating, scary, or painful for your dog, it's time to say, "thanks, but no thanks," and call someone else.