Refusal to eat
Whistling or wheezing while breathing
Crackling or popping noises near the snake's mouth/throat
Drooling
Mucus building up in the snake's mouth and/or throat
Mucus smeared around the sides of the enclosure
Snake rubbing its face against the sides of its enclosure or "furniture"
Snake resting with its head propped up at a sharp angle
Those are the ones I can think of. They don't all come at once and may be seen in any number of combinations.
