Ask if they have a
care sheet for bearded dragons, and if not then ask which
care sheet they recommend. Take a look at it and make sure everything suggested on the care sheet falls in line with what you know to be correct. Ask how many bearded dragons they've seen within the past 5 years. Ask if they've ever done a blood draw, x-rays, fecal tests, ultrasound, and/or surgery on a bearded dragon. Ask how familiar they are with things such as yellow fungus and Adenovirus. If they are well versed with the majority of these things, then I would consider them a good herp vet option. If they don't have much direct experience with beardies, but have worked with other desert reptiles instead (like uromastyx, etc.) then they may still be an option worth trying.
I also entered the name of my city into the search bar function here on the forum to try to find other members who lived in my area. Then I PM'd them all and asked for vet recommendations.
I understand your frustration... I tried and paid for two different "exotic" vet appointments before I finally called or emailed every vet in town to see if they could answer these inquiries adequately. I was honest and explained that I was tired of wasting my money on vets who didn't have a clue what they were talking about. If the vet is legitimately knowledgeable, then they shouldn't have any problem answering these questions for you before you make an appointment. Hope that helps, good luck!!
P.S. I consider my beardies' primary care resource to be Tracie Kretzschmar (username Drache613 on this site). She is not a veterinarian, but she has a year or two of vet school under her belt and she is more knowledgeable about beardies than any vet I've come across. She does phone consultations for a small fee (check her out at
http://www.bug-de-lite.com/) and she knows how to evaluate blood work, x-rays, etc. plus she can recommend/source medications and tell you how to properly calculate dosages. I've had her evaluate every test my vet has ever run on my beardies, and she's caught things that my vet overlooked - even though I consider him to be a great herp vet. I usually schedule a 30min-1hr phone consultation with her once every couple of months. She even heads up a big Adenovirus research project and has access to a lab that you can send a fecal swab to get your beardie tested for it! She's definitely what I would consider a bearded dragon "expert", and is a great resource for your beardie's health & care.