This is like asking how do you tell if a painting is beautiful. There are many levels to experience. A person could memorize 100 books on all levels of husbandry but until you see it a few times its hard to notice the differences in the animals body language that indicate, they are hungry vs just being out of their hide. I still see my BP head out and think tonight she will eat only to have her recoil as soon as the rodent is presented. The following week I think ok what was I thinking last week. That's a hungry snake and sure enough I get a good feed. In 10 years I might be nearly perfect in reading my snake only to fail miserably when I get a new BP. Time and number of animals can help one person gain experience while another learns nothing. I have far more experience with horses and the one thing I have learned is that a person is only really good at judging whether another person has more, less or the same experience level as them.