Everyone is a critic. If you have a job specialty I am sure that you look at others performing your job with a critical eye. Maybe they are less efficient, charge too much or have a questionable work ethic. I am the same way. I am always critical of other dog trainers and enjoy watching other trainers work to hone my skills of assessing a situation and formulating a strategy that I might do differently.
I first put my criticism into two categories: humane and non-humane, and then I filter through many other factors including clear instruction, efficiency, mastery of subtleties and speed of learning. The first one is the deal breaker.
If I ever hear of, or see someone performing training that is not humane, I lose complete respect for the trainer. There are many levels of inhumane treatment of dogs in dog training and the important point to keep in mind is that aversion, or the application of something that is unpleasant, is in the eye of the recipient. Some dogs might be fine if you raise your voice, others might cower and shake in fear.
The point is, it is impossible to know the threshold of pain and anxiety that a dog can take before they turn into a mess, so why apply any level of aversion? There are numerous trainers that use much more abusive trainers including using choke chains and shock collars. The tragedy is that the dog training world is largely unregulated and clients hire "dog trainers" because they don't realize that these abusers are using methods that can physically and emotionally scar their dog.
Read this story about a dog in the UK that was tortured by someone calling himself a dog trainer. Thankfully, this poor excuse for a human being was found out and fined, but many much more heinous actions are being done RIGHT NOW to dogs all over. Just because someone takes your money or has a facility with a cute sign with a dog on it doesn't mean that you should trust them with your dog.
Do your homework, check references, watch them work and DO NOT get scared into using their services. Many abusive trainers use the same scare tactics on their potential clients as they do with the dogs in their care. Especially if someone has an aggressive dog, they often tell them that if they don't "Show their dog who's boss" and "Be the alpha" and "Whip them into shape" then their dog will never get better and will probably get worse. This simply isn't true. I have worked with thousands of dogs, many of them were aggressive, and have only used positive reinforcement.
I will NEVER use choke chains, prong collars, shock collars or abusive methods. Keep this in mind when you are looking for trainer. Just like finding a good teacher, finding a good trainer might take some time. But it is worth it.