I can never, ever, ever tell when my avalanche is going to shed. (He's the same color as a snow). One clue is that he's reluctant to eat- he's never refused, but he takes longer and sometimes needs his mouse reheated back up to really hot. Another clue is he hides away, but he barely comes out anyway, so that isn't the most helpful. If I catch him at the right moment, his eyes have a pink opaque color, but it doesn't last long at all. Usually I just find the shed after it's all over.