Anyone can "be a trainer".
Unlike professions such as doctors or lawyers, where you gain an education and then are required to pass a legal test in order to provide those services within an industry, there is no standardization when it comes to dog training as a career. Anybody, at any time, can open a business and claim experience as a "dog trainer".
"Having dogs all my life" is not an adequate level of education to give you insight into the minds and behaviour of animals (although if your trainer is currently raising his/her first dog, also take note). Nor does it give you any real guarantee that a person is actually able to teach dogs. As I heard from a wonderful person once (I can't remember who, my apologies!)..........I have had teeth all of my life. But that doesn't make me a dentist.
Also be aware when trainers tell you that they have gone to "dog training school". Please, don't take that at face value. I can easily spend $400 and a nice company in another country or province will mail me a beautiful certificate saying that I read a few books, passed a multiple choice test, and am "now a trainer". Heck, some of them will even send you the certificate in advance! Reading books doesn't make you a good dog trainer - training dogs does. While there are some great organizations out there that are doing their best to make dog training a more professional industry (APDT, CPDT, CAPPDT), with certifications using academic exams and proper protocols for certification, please do your research and look deeper when somebody claims "dog training school" as an educational background. A lot of the time it doesn't actually mean anything. As a trainer, I feel that is important for clients to know. Please dig a little deeper when somebody claims they have gone to "dog training school", as not all programs are of the same quality.
So, what should you look for? This is not a complete list by any means, and you can often find a rare gem who does not fit the bill, so to speak, but it should give you an idea.
Educational Background. Good dog training principles are based in sound science. While the actual teaching of dogs and application of methods is an art, the concepts used behind them should be sound, and have some basis other than hearsay and folk tales. Look for folks who have studied in an academic setting, such as an accredited college or university, whose degree or diploma have relevance to the field (psychology, ethology, biology, agriculture, and animal sciences are good places to start). It's a good place to start.
Continuing Education. The beauty of dog training as both a science and an art is that we are learning more about our canine companions all the time. Every year, we develop a better understanding of how dogs learn, what motivates them, and how to teach them. If somebody tells you that they know all there is to know about dog training, please halt there. Continuing education is a key component of humane and modern methods in teaching our dogs. "Even trainers have trainers" - that is, we should all strive to continue to learn from one another, and from testing our ideas and observing the results.
Actual Experience. While a degree or diploma is great, it takes more than just book knowledge to successfully teach dogs. In order to be successful at it, you need to practice it. Lots. Look for folks who are involved in foster care, rescue work, assist at a shelter/daycare/boarding facility, or are active with their own dogs (often trainer's dogs can give you a good idea of the level of skill of the trainer.......if they have dogs with poor manners, are they the best candidate to teach you or your dog?), or have shown in some way that they have the mechanical skills to teach what you need help with.
On that same note, look for relevant experience. If you need help with serious behaviour problems with your adult dog, perhaps using a trainer who only holds puppy classes may not be the best choice. Or if you are looking for an agility class, finding somebody who has trained their own dogs and actually competed in agility trials and understands the sport in depth, may be of more help than somebody who has trained their own dogs on a few backyard obstacles. Don't be afraid to ask questions and find out if the trainer you are seeking has the experience you need.
Honesty. What I mean by this is that good trainers are honest about their strengths and weaknesses. Nobody "knows it all", as we've already mentioned. Trainers should be honest when they are dealing with an issue they are unfamiliar with, or are uncomfortable with. Good trainers, when they don't know the answer, won't make one up - they will find the information to help you, or they will refer you to somebody who has more experience in that field. Because there is such a vast array of information out there, and because nobody has all the answers, good trainers have networks in which they can discuss issues with other folks to be sure they are providing the best services possible.
So please - when looking for any trainer, look for somebody who is interested in helping YOU and your dog, and who works hard using dog-friendly techniques founded in science (not folklore or opinion! Take caution when you hear trainers claiming terms like "alpha", "dominance", or try to sugar coat painful tools as non-painful accessories) to help your specific situation, not a cookie cutter approach that may not be at all what you need. A good trainer should be helping fix the relationship problem and make life better for both you and your dog - not just put a band-aid on the symptoms regardless of the effects it may carry out for your furry friend.
Every dog is different than the next, and every family is different than the next. Choose wisely a trainer who can reflect that without hurting your dog or relationship in the process.