As our dog behaviourist friend,
The Dogfather will tell you, there is no real recognised qualification for 'behaviourists'.
There are associations that you can get certificates from, but some of the ones I personally know of are run by a handful of people who one day decide that they're going to call themselves 'The Association of UK Dog Trainers' or something similar and take peoples money to award them certificates.
Dog Training is extremely unregulated. Anyone who I know who has these qualifications are usually the worst dog trainers going. Dad actually did one of these courses to become a 'Certified Security Dog Trainer' with the biggest Security Dog Association in the UK but they hardly did anything with an actual dog the whole time he was on the course. All you have to do is tick a few health and safety boxes and then they take your money and give you a certificate and thats it.
He hated it.
In my opinion the best indication of a good dog trainer is one that competes regularly in some sort of dog sport (and does well) and shows videos on their website of what they can do (but of course I'd say that

).
Also, they should always be prepared to get their dogs out and show you in person when you get there what they can do. I know lots that never get their own dogs out, for good reason, they're crap!