While I am new to chameleons, I am not new to exotics (parrots). Finding a good vet who can deal with your exotic is really important and you want to find that vet well before you need them.
I'm lucky that I already have an avian vet who also does reptiles. In general, birds have a lot more in common with reptiles than they do with mammals, so many avian vets--unless they completely limit their practice to birds--will also treat reptiles. Birds, after all, are the last of the dinosaurs. Seriously.
If you don't already have a relationship with a vet that you believe is competent to work with your chameleon, my suggestion would be to find one now and take your chameleon in for a wellness exam. Other than the fecal, I personally think a wellness exam on a healthy chameleon is a waste of time and money, but the important thing is to find your vet and start developing a relationship with him or her before you have an emergency. Vets are a lot more accommodating to their clients than they are to a stranger at the end of the phone on a Saturday night.
I don't take my parrots in to the vet unless I have a problem, and surprisingly, the parrots have never gotten sick in the 15 years I've had them, and one of them is pushing 30 years old. Various chickens I have, on the other hand, have been to the vet countless times.
Monitoring stool is really important. That will tell you a lot about the general health of your animals.
When deciding to go to the vet, sooner is always better than later. Like birds, if you notice your chameleon is not right, it is probably really, really sick. They can be close to death, but if you try to handle them or catch them, they will try try to escape and struggle as if they are well.
I'm quick to go to the vet and don't begrudge him his consult fee, even if it is a lot more than the value of the animal I am bringing to him. I've even taken favorite chickens to the vet to be euthanized even though I will slaughter extra males. Go figure.