There are particular qualities of temperament I want and I look for indicators. A lot of things done in puppy selection can still be done with adults and there are some tests you can do on semi-adults that you shouldn’t on a puppy. I learned a lot many moons ago reading up on guide dogs for the blind puppy selection. A dog desired for protection should display some traits that would be undesirable in a dog desired to never display any amount of aggression. The last time I evaluated a dog for my own, it was a young male GSD who’d been picked up on the street as a stray, in a shelter until his kill day, pulled out by a herding breeds rescue group who did not have a place to put a large dog, handed over to a GSD rescue who immediately sent him to the vet overnight to have his bits snipped off. So a week or so after he’s taken off the street, I show up to eval him to come have a forever home with me. I’m looking for steadfast who will have what it takes to protect me on trails.
The only fair test I can do is a startle. I have a large coffee can with a few metal spoons inside. When he turned his back and had his attention diverted, I slammed the can onto the stone floor behind him. He turned around with his ears up, interested and immediately went to the can to investigate - no shy at all. OK, steadfast fella, come be my boy.
A decade later, he has over a dozen cougar encounters, five bears, bob cat, never engaged, just closed the trail, told me in plenty of time for me to turn around. The horses obviously got his job. Verr cool.