The first and earliest sign is always reduced appetite with associated weight loss. This sometimes shows up as chronic constipation. Some chinchillas will go off their hay first and favor pellets. Others like mine did not seem to favor pellets but did have weight loss. His front teeth were maloccluded. This is going to be different if it's a back tooth since hay must be ground down by these.
The best thing to do is establish what is a healthy weight for your pet and weigh him regularly (about once a week). If you see weight loss of 20% of his normal or healthy weight under constant conditions and calm environment it's time to see a vet. If he's had an exam and there is no other reasonable explanation for loss of appetite or anorexia it's time to get a dental exam. Other common symptoms are drooling, foul smelling breath which can mean an infection, pawing at the mouth and apparent agitation when eating (mine had this symptom although it was hard to catch), bloat or stomach pain. Less common symptoms are watery eyes (which usually occur at the end stages), or infection under the jaw from roots growing through the skin. Usually animals die from anorexia before these last symptoms occur.