I had a wonderful trainer for our puppy classes - she insisted on the puppies not meeting each other at all, or at least for no more than three seconds at a time. Of course this rather went out the window when it turned out later that one of the dogs was going to the same daycare as Bill, and in fairness he also made strenuous efforts at the time to climb every barrier to get to the other dogs.
These days our issues are mainly around his over-arousal: I think it's social competitiveness with other entire males, rather than a fear response, but I don't know for certain; and he is a bully with puppies (and has been since he was one). I spoke to the vet yesterday and got a referral to the University where I work myself, which has a really good dog behaviour clinic - frighteningly expensive, but my insurance should cover it. The vet agreed that we'd be better off trying to deal with it now than waiting in the hope that the end of adolescence would resolve things, when that means at least three more months of him practicing these behaviours.
When I look for trainers, the first thing I check is that they have qualifications and memberships of professional organisations, and then what those are. E.g. IMDT, Pet Professional Guild. I'm also keen on personal recommendations. But actually, a lot of what you need to know is between the lines - e.g. I came across someone recently who had been an academic but had set themselves up as a dog trainer with only a handful of courses she'd taken online from a trainer who's known to use aversives.
I won't touch anything aversive with Bill, even when he's driving me completely round the bend, so I don't want to work with someone who thinks (or might think) they're reasonable tools. There are some which are on the border I think - slip leads are common in gundog work as a dog going into cover could get a collar or harness caught; and even DTAS have one head collar (Dogmantics) which they list as recommended. I don't want to use either. I'm not sending Bill into heavy cover to hunt anything, and I'd rather he had a collar and ID if he ran off anyway. And his pulling on the lead is down to over-excitement and I can manage him even if it's sometimes unpleasant (different with Brie given her size).
I don't agree on everything with our current 1-1 trainer, but we are probably 80% aligned, and that's fine. We both like using food but she would probably suggest I use toys more. She doesn't use aversives, she makes a fuss of Bill when he bounds over to her, and she has enormous patience and understanding of his nobheadery. She's not (yet) a specialist gundog trainer, but there's nobody like that within reach for me that I'd trust, given he struggles with over-arousal in the car as well. I'm only really adding in a behaviourist as what we're doing with her is giving me skills to help him use his brain and instincts, whereas what I need from a behaviourist is how to change his emotional response and dial down his levels of arousal, and those aren't quite the same.
@CoubousAndTourmalet do you know anyone else who has the same breed as Brie who might be able to recommend someone? Most of them do online sessions these days too.