To begin with, use all of your resources, whether it's friends with well-trained golden retrievers, your golden retriever's veterinarian, golden retriever magazines or the internet. The more sources of information you have, the better off you'll be.
Once you've compiled a list of possible golden retriever trainers, the next step is to contact them to set up interviews. If possible, bring your golden retriever to the interviews with you. You want the interview to be a question and answer session, not your golden retriever's first training lesson, but having your golden retriever at the interview can give you a feel for how your golden retriever and trainer will get along together.
During the interviews, don't be afraid to ask questions. Ask the potential trainers about their training methods, how they deal with problems such as golden retriever aggression, how long they've been training golden retrievers and anything else that you feel the need to ask. Be sure to pay attention to not only their answers, but also their body language and the tone of their voices. After all, it's just as important that your golden retriever's trainer get along with you as it is that they get along with your golden retriever.
If possible, try to observe a training session with another golden retriever or ask if you can talk to a previous customer of each potential trainer. What you may expect and what actually takes place during a training session may be two completely different things. This added insight will help you to make an accurate final decision.
In short, finding a good golden retriever trainer depends on many things. There is no clear right or wrong answer. If you do your research and follow your instincts you should find a great trainer to suit your needs. However, even after you've chosen a trainer, don't be afraid to ask about things that you don't understand, question techniques or ask for a second opinion. You and your golden retriever will be glad that you did.