Drooling, a lot of sticky saliva, yellow or white stuff at the corner of the mouth. Thin looking ribs and pelvic bones showing through the skin. Skin is wrinkled and dry, not supple. Does not change color when picked up. Does not react to lights and movement around the cage.
Chameleons hide signs of illness until they are very weak.
By the time many of these symptoms show up, survival is poor. Veterinary treatment can be difficult and expensive. It is best to buy a healthy animal and do all you can to keep it that way. If you don't know what a healthy one should look like, ask an expert for help. Think carefully about why you want this animal, whether you can give it a proper home and whether you can devote the time and attention it will need.