Trainers and Dog Aggression

As a dog trainer for over 20 years my best advice is to start by asking your rescue group who they suggest, your vet, friends with well behaved dogs. I would make sure the trainer has experience with as many breeds as possible and rescue dogs.
Ask what methods they use. If they say I only use food, or I only do this or that, keep looking. You want a trainer with many methods and expereince ( thousands of dogs not hundreds)because you will always find a dog that is the exception to the rule and your trainer better have experience with more than one method. Ask what breeds of dogs they own and trained. One or two different breeds is not enough. Training only goldens is way different than have trained terriers or work group dogs. Remember it is about 12 weeks for a dog to understand and be retain training. So don't be in a hurry and disregard 6 to 8 week training program, you will still have to work at home to reinforce those skills. Ask to sit in on a class and see what you think. Does the trainer have control of the group? Ask the other students what they think. Call former students, no one should be afraid to give you references. You need to understand your dog is not as well trained as you precieve it or it would listen to commands even when other dogs are around. So reestablish the basics and it will help many problems along the way. It is never ever to late to socialize and retrain, most people don't understand that its the whole big picture not just the aggression snapshots. Aggessive because under socialized, breed of dog, fear of other dogs, a good trainer will answer that question and know how to solve it. So be open to trainers suggestions, which may include a group class to put your dog in a stitution where it must be around other dogs and behave. Good luck.