As a breeder and what i believe in, is making sure any animal that leaves me, is in the best shape it could possibly be, and i hope thats the way everyone approaches breeding (but just in case) 12 week old beardies tend to have a pudgy belly (unless they have just emptied there bowels) you don't want to see ribs, they may look flattened out when basking but should not look like a pancake!
usually bearded dragons have between 2-3 clutches after mating with a male (2 is more usual then 3) Ask to have a look at a younger clutch if there are any, the 12 week old beardies should be a reasonable size bigger then the younger ones. some clutches do better then others. Healthy female = health babies, if the female doesn't look healthy that a good indicator that you may have problems down the line with her young. Sometimes your timing may be off, asking to see them eat isn't always reasonable to ask.
But a dragon that doesn't eat, will be smaller then their clutch mates (there will always be Dominant and less domination individuals even at that young age). A good sign of underfed dragons, is if there a loss of toes or tails within the clutch. Clutch mates will attack other dragons thinking that they are food (worm or cricket) well fed clutches will not have these problems. Young dragons don't tend to get skinny, unless drastically underfed, they just wont grow as fast as ones that get more food.
As a rule, a healthy animals will look.... Healthy and they are hardy animals.
Make sure he's eating veg also, not just crickets. Veg is important.
I personally have NEVER had a dragon that refuses to eat for me, one of my 8 year old females did, but this was a few days before she passed away.
Good idea going to a breeder though, i support pet stores, they further our community and there wouldn't be much of one without them. But from experience it not likely you will walk into a pet store and be passed a baby that they can tell you its exact age, breeders however should be able to.
Good luck with your new addition, have fun, love and care for your little guy/girl and im sure you will be just fine! Not to mention you have a forum full of people willing to help! Keeps us updated and the more questions you ask the better, dont feel as if a question is stupid because there are no stupid questions when it comes to a living animal.