Your point about how a specialist can connect their knowledge to what they see is true, but not relevant to the point at hand. If I was your therapist, and had studied and understood you thoroughly, I might be able to assess your mental state at a glance in most scenarios. That doesn't deny you have a detailed inner life anymore than that an arborist might generally understand a tree does not deny the complex life of the tree.
Yes you can do that. To put it extremely simply: define what "friendly", "sick", and "lazy" means in terms of behavior, then observe the dog's behaviors.
Edit: sleeping is a behavior.
?? Obviously you can. Why do you think you can't?
you can't identify every friendly dog just by looking at it, you'll identify dogs that are also acting friendly and doing things people have found preceded positive experiences.
you can't identify every sick dog just by looking at it, otherwise the vet would never find additional issues.
and so on
Yes, it's wrong to assume that you can tell everything about that dog by looking at it - but it's so much more uncomparably wrong to consider that you can tell nothing about friendly or sick dogs based on looking at them!
Observating the behaviors and dog breeds correlating with previous friendly and unfriendly experiences is a very useful, somewhat reliable predictor of how likely this particular dog is to be (un)friendly. Sure, if you know this particular dog, then that should supercede any group information, but if not, then that's all the information you have, it is useful information that correlates with (future) reality that matters to you, and so it's prudent to use it. Appearing "nonjudgmental" by throwing away information and judgment is simply stupid, and bound to get you bitten at least in the metaphorical way.
but nobody in this entire thread was saying that, and my contribution was an additional attempt to further point out why the other sibling responses were missing this
At this point I'm totally content in talking past each other on this topic as I honestly don't know why its not clear that the subset which is undetectable by mere observation is statistically significant.
When an arborist "looks" at a tree, they identify the kind of tree it is, and that connects to all of the research they know about that species of tree. It's reasonable to suppose that a new instance of a known kind of tree is going to have properties in common with all the other known instances.