The first thing I'd look for is a vet listed as an exotic-pet veterinarian because they're more likely to be able to care for reptiles than a normal veterinarian would be.
It might be a different case in bigger cities where vets see a wider variety of animals, but I've not been impressed with the knowledge of non-exotic vets regarding animals other than cats and dogs. I believe this to be an issue of demand, which might change as reptiles become more popular as pets, subsequently being considered less exotic.
Most vets probably do have some training with reptiles, but I wouldn't expect it to be as in-depth as a vet specializing in exotic pets. There's also the matter of how often they see reptiles compared to how often they see cats and dogs. Even if trained with reptiles in school, if practicing on a reptile is a rare occurrence, their reptile training might diminish due to lack of use.
Asking the obvious question, if they're trained to care for exotic pets (specifically reptiles), is best. It should result in an answer of either yes or no. If the answer is no, you should follow-up asking whether they can recommend a properly trained vet in your area.
I don't know of any vet who would lie about this, as (as it is with human doctors) it's a pretty big risk that if something goes wrong, they can be held liable for it. If you're worried a vet you're seeing isn't as capable of caring for reptiles as they say, ask a couple follow-up questions about their comfort levels with reptiles.
- how many reptile owners see them for appointments
- what kind of reptiles do they usually see for appointments
You're going to have to use your best judgement to decide if you want to trust them.
If no vets in your area are trained to care for your pet, that doesn't necessarily rule them all out. A vet who is willing to learn and consult a specialist or reference manual if needed, is a much better option than not taking your pet to a vet at all. It all comes down to a matter of using your best judgement on which vet to trust.
With all that being said, I would say the best way to find vets who are qualified to care for exotic pets is by searching through the Association of Reptilian and Amphibian Veterinarians' website. They have a search feature for certified vets by state. Likewise, there is an Association of Exotic Mammal Veterinarians, and an Association of Avian Veterinarians if you're into those types of pets.
If nothing else, ask around. Find a local group of reptile keepers or your pet store, and ask them which vet they use and trust. It is the same as finding a vet for a dog or a cat; hearing personal recommendations is always helpful.