What other test is there for things like poise in the pocket, good decision making, having the arm talent to make top level throws, having the mental makeup to make those throws even with the D in great position, etc.?
If you don't like the eye test on him, feel free to look at the stats from Monday night.
FWIW - I have never been fervent fan of either Trevor or Fields. Herbert is bona fide. If you can't see his talent, might want to get your eyes checked. Before anyone chimes in with any silliness, his organization has been crap & he's had some unfortunate injuries but his talent is recognized by players, coaches, GM's as being up there with Mahomes.