It really depends on the issues that your rabbit may be having. Typically an initial check of the teeth will be done consciously with an otoscope. If there are definite signs that it is a dental issue such as drooling or odd chewing behavior, or if nothing was found in the conscious exam but the rabbit is continuing to not eat, selectively eat, or other symptoms that may be dental related, then the vet may want to do a more thorough check under GA.
With xrays, they aren't going to show the vet problems with molar spurs, but are more to try and detect infection, irregular tooth growth, or elongated tooth root issues. Sedation of the rabbit is usually needed so the vet can get good clear xrays of the head. If a vet does a dental exam under GA and doesn't find any spurs or visible signs of dental problems, then it's usually a good idea for them to then do some xrays, as the issues causing selective eating may be below the surface, such as tooth infection/abscess or elongated roots causing pain when your rabbit chews.
Rabbits can have molar problems even though the incisors are perfectly normal. The reason for this is that incisor wear depends primarily on whether or not they are aligned correctly. If not, the rabbit will continue to have overgrown incisor issues. But with molars, their wear not only depends on proper alignment, but also getting enough tooth wear from the food the rabbit chews. So even though the incisors are perfectly normal, the molars can still develop spurs if they aren't getting worn down from roughage in the diet.
Has a vet checked his molars? The selective eating can certainly be a sign of dental problems. Especially if he used to eat hay fine, and if you have tried changing to different types of hay to see if that was the problem. One good test is trying a bit of alfalfa hay, stems and all(if no bladder sludge or kidney issues). Not too much though as it may cause digestive upset if a bun isn't used to it. Most buns really like it and will eat it when offered. I had a bun go off his hay, and pellets too for the first part of the day. I tried a couple different kinds of grass hay but still nothing. So after 3 days of pellets/veg only and no hay, I was starting to think dental issues. Then I tried a little bit of alfalfa hay. He ate it right up and after a few days was eating his grass hay again. All I could figure was that he got something in his hay that upset his stomach and so that put him off of wanting to eat hay for a few days. If you try the alfalfa, if your bun will only eat the soft leafy parts and not the stems, that could still mean dental issues, but if he will eat the stems, it may mean something else is behind not wanting to eat hay. Figuring out what is the tricky part.
If your bun is found to have spurs, one thing that may help with the wear of molars, is orchard grass, if available to you. It has a high silica content that is good for wearing down the teeth. I know of one lady whose rabbit was having regular dentals, I think it was every 6 weeks. She started feeding her bun orchard grass and last I read her bun was up to 6-8 months without needing a dental.
This link has some detailed info and photos of dental issues in rabbits that may be helpful to you. It has some good xray pics of some dental problems.
http://www.manorvets.co.uk/ckfinder/userfiles/files/dental%20dz%281%29.pdf
