I can tell if my snakes are asleep in the following way.

I pull out their tub, grab them and then for a second they don't move and then suddenly give you the "OMG WTF BBQ, WHAT IS GOING ON I WAS FRIGGIN SLEEPING" stance. You get used to it. In fact usually when I grab my snakes they tend to be asleep, it's rather funny and at the same time I tend to feel like a jerk.

Easy way to avoid waking them up is to only go in and grab them when they are moving around.