Good points.
I also look at the kid and the time of the season as well. Some kids work on throwing all offseason so they are much closer to midseason form than other kids that need to work on their arms.
Some clues that I tended to look for with younger kids is an increasing lack of control. This seems to be the first sign of a tiring young man. The kid may still be able to throw hard but the muscles that control the fine motor aspects of throwing seem to tire out more quickly and that is when I worry.
When I was coaching I had an absolute pitch count of 80 on the kids, at mid season. Some boys had less but none, even the ones that were horses, had more. Early on, 60 tops. This is pre high school level, remember this. The goal to me was always to teach the kids and let them have a good time and winning was secondary, especially at the 12U level.