Experience, Positive and Pro active!
EXPERIENCE - Some trainers will say they can fix anything and some trainers think they can!!! Real trainers know the difference between a training problem, a problem behaviour and a behaviour problem. A good trainer also knows the limits of their own expertise, and does not want to make the problem worse by giving incorrect advice or ignoring a potential illness. These trainers will refer you to a Veterinary Behavioural Specialist if they feel that it is best for your pet. Be wary of trainers who claim to be able to fix everything. Good trainers, Veterinarians and veterinary behaviourists often work together. A trainer will help you with the behaviour modification program that the behaviourist has recommended. You want your dogs trainer to have many years of experience and the maturity to deal with your situation should a referral be required. You also need your trainer to know that not all behaviour is as a result of lack of training some can be due to illness, pain and much more!!
Positive and Proactive - Don’t hesitate to ask your potential trainer about his/her training methods and continuing education experience. Many training methods which were considered acceptable in the field years ago have been found to be counterproductive methods. The TV shows that we see everyday are also showing unproductive training methods. Positive trainers have a lot more to offer you and your dog - consider how you would like to learn something new - By Force and Aggression or by Rewards and reinforcement?
Remember that there is no required curriculum nor is there an entity which regulates dog trainers; and, basically, anyone can call themselves a dog trainer. Ask your friends, your Veterinarian, Groomer or Dog Walker / Minder for a recommendation, word of mouth is by far the best then you know what sort of results to expect!
It is, therefore, up to you to be proactive and do your homework when choosing a trainer.