There are some pretty valid reasons for dentistry on the Mexican side to be cheaper -- differences in how dental training is structured and paid for, lower wages, etc. -- so I think you can get a good healthy deal there. But for heaven's sake do your research -- ask lots of questions, including about the credentials of the person who will be working on you (not just the person who owns/runs the clinic), read lots of reviews, take advantage of the free-consultation offers so you can check a couple places out, and if at all possible get a personal recommendation from someone you know and trust. (Or if you don't know and trust them, ask if you can examine their teeth. LOL kidding!)
Not really all that different from what you'd do if you were shopping for a dentist on the US side, as RylieSutton said -- you just have to do a little more of it, and it's a bit harder to do because the whole scene (professional associations, Better-Business-Bureau-type resources) is less familiar -- but it's do-able.
If you google terms like "medical broker" or "dental broker" you'll find all sorts of organizations that exist to help you find health care in Mexico and other countries. How reliable they are, I don't know. But at least they might be a starting point. I assume (hope!) that they at least verify training and licensing. You could look on a website like that to get a few names and then research them further. One caveat: the reviews on those broker websites seem like BS -- 99% five stars, better than a North Korean election!
It's not all that unusual to find dentists with US training and/or membership in US professional organizations -- sometimes they even live on the US side and commute to Mexico to work. It's common for at least some staff members to speak English (or at least it was where I went) -- and many clinics seem to have a US phone number as one of their contact options. So if you find the whole due-diligence thing daunting, you could limit yourself to that group for starters.
Good luck!
