how does one get involved in that sort of thing?
And, a fair amount of experience in that class of airplane is pretty important too!
With considerable experience in advanced GA singles, particularly amphibians and flying boats, I used to do a number of checkouts, and found exactly what BPF has observed:
The problem is that I usually find multiple areas of poor flying skills, so it isn’t a checkout it is basic flight instruction because I am not going to sign off on anyone without good aircraft control and I am not really interested in doing basic instruction anymore.
it's tough enough to refresh the candidate on basic flying skills on a basic airplane, but attempting basic training in an advanced or otherwise complex layer is more difficult. On a few occasions, I have anticipated this need, and recommended (though it was not really a choice for them) some refresher training on something simple first. Two examples have been a request for a checkout in a rather demanding taildragger with no tailwheel time - I sent the candidate to a well known taildragger trainer in the US to get a "tailwheel endorsement" first. In another case, prior to training a new owner who had earned his PPL on floats, in his new amphibian floatplane, I send him for a "landplane endorsement" in a 172 first. In both cases, I think it made the difference. Had either resisted my suggestion, I would have declined the training.
There's a lot of responsibility on the training pilot to get the training correct and complete. It's stressful to send the candidate solo in the advanced plane, and hope they can demonstrate the skill safely. I got that wrong once on the landing just before I was going to send him solo - we both ended up in hospital. This past summer (in exactly the same spot in the lake as my accident) a new pilot, on his first solo flight after type training wrote off his plane, though survived. From he video of the landing I saw, he was certainly not completely ready for solo flying in that plane. Conditions were not challenging, and he still mismanaged things.
The check pilot has to have the guts to tell the candidate that they are not ready , and the candidate has to have the spirit to hear and embrace that wisdom! Decades back, I trained a new owner in his Bellanca Viking. After 19 hours of training him, I could not see that we would ever get there. I got up the nerve to break the news to him, and he was gracious about it.
A couple of pilot friends of mine, whom I consider to be "mid skilled" have mentioned to me their desire to be check pilots. I have cautiously not encouraged that for them.