With Emeralds, I have seen many perk up right before death. They are a weird animal (probably not what you want to here). Two major things will really be an indicator of the animals health. 1) is it perching correctly. Tight coil, head tucked on top or in center, and 2) when you mist it, does water bead nicely on its skin. In regards to food, the best clue that your snake will eat, is that at night it will perch with its head downwards. This is a sign that it is waiting for a rodent to walk past and a sign of a willingness to eat.

These can be affected by health, but also strongly by the enclosure set up, heat, humidity, etc.

Warren