Look for reduced respiration, lack of tongue flicking, and a general level of not being aware of the environment. See those things, and the snake is asleep. Sometime you think, "Is it sleeping?" but usually 9once you get a feel for it), you can often tell when on is asleep by telling when it wakes up; startle it, put the smell of food near it and watch it START to react to the environment, etc. Snakes are easy. I'm not surewild bullfrogs EVER sleep.....lol.
KJ