A good place to look is the ABTC website. This is an umbrella organisation that only allows organisations with assessed members to an agreed standard. Those standards include practical, written, interview and thus require a demonstration that the trainer has been in the sector long enough to have all the necessary skills required.

But don't rule out trainers that are not on the ABTC website, there are some brilliant trainers who are not members of the ABTC, who have been assessed by other associations and/or have years of good experience, You should still, when choosing them, be able to see demonstrations of their experience in all areas.

You can always quantify with the prospective trainer if their wording is vague. For example, if someone advertises "I've always had dogs" you could ask them to clarify. Do they mean they had 5 German shepherds growing up or they bred hundreds of them and worked them? Even in the latter case, it is worth finding out if this experience is complimented by scientific study before deciding if they are the right person to be working with you and your dog.

Another example, driving a car is not the same as being a mechanic, so if someone were advertising that they walk and train dogs, you could clarify what percentage of their experience is walking and what percentage is training. 

A similar logic would apply to puppy parties. A dog's personality is being shaped in those tender young ages; it is the easiest time to create behaviour issues. Puppy socialisation is therefore best carried out by qualified trainers. There is a lot more skill to it than meets the eye and just a few minutes of distressing experience could imprint reactivity issues for life.

Even at the hands of another friendly puppy, who didn't even realise they had scared your dog. Of course, anyone can supply this service, but logic would beg the question, why would you not get a dog trainer to do this.

A word on taking advice from friends, family, someone on the street, at the park or generally anyone who has only experienced a few dogs. They will only be able to give you advice on what worked for them with their dogs. This probably won’t apply to or work on your dog.

Similarly, TV shows are edited to show that the training worked in a half an hour episode, by all means watch and enjoy them as entertainment. But that is all they are, entertainment. Dog training just like any other skill in life takes time. TV producers tend to gravitate towards apparent quick fixes often with aversive methods, as it makes the best TV. You would not attempt a medical procedure on a family member after watching Holby City, and whilst with dog training the stakes are lower, the analogy is near identical.

Another thing worth finding out is the trainer's methods. Are they positive? Generally speaking, Positive would mean using methods that do not expose the dog to unpleasant (aversive) things. What might be considered aversive? I can suggest the following list of things I would not use, as I would consider them aversive: Choke chains, prong collars, electric shock collars, yanking the dog by the neck, spraying the dog in the face, routinely shouting at the dog.

Yes, we can all get frustrated occasionally, we are all human and will occasionally tell the dog off. However, we do not need to use this as routine training methods, as there are professionals available who can show you how to train your dog without these tools. You will obviously need to decide yourself whether you go with a trainer that uses positive reinforcement or whether you are happy for someone to come and apply those aversive tools to an animal member of your family..

On top of all this experience, I also have a network of dog training mentors to ask something when I need to know more about a subject or don't know something off the top of my head. Yes, sometimes a professional might not know something and need to find out. This is part of being a fit and proper professional, so beware anyone who says they know everything about a subject or only has 1 method at their disposal. A good dog trainer should be able to flex and mould with the dog and owners at hand.

Below is a very famous chart which plots the knowledge level of someone working in a complex field. This can apply to more obvious professions like doctor's and in this case canine behaviour. Canine behaviour is a complex mix of genetical traits, nature vs nurture, experience, hormonal biochemistry, medical causation and lots more. Once you realise how many of these areas are relevant, it soon becomes apparent that training and behaviour can even be more complex than medicine. Animal behaviour even more so, as they cannot speak to us.