I'm looking at some used guitars. I can pretty well gauge a guitar's condition, etc., but I'm wondering about the inside. I can't see inside. Suppose there's a loose brace, but not loose enough to maybe rattle or something. Maybe a brace fell off and now there's a missing brace. Or anything else that might be wrong inside. I can't start taking off the strings and probing around. 1 )Any way to inspect the interior, and 2) What should I look for in particular.

In the end, the sound is the sound, and I guess if something's wrong inside there are only two possibilities. 1) Whatever is wrong doesn't really affect the sound, or 2) it does affect the sound but I'm perfectly happy with the affected sound. Or there may be a third possibility, which I guess may be the one I would be worried about – that something's wrong inside, but it hasn't quite yet progressed to the point where it will have a deleterious effect. Then after I buy it, some time later the worst happens.

Appreciate any thoughts, especially from our luthiers.

Usually if there is a loose or broken brace you will know it from bad vibrations. I use a mechanics mirror about 2 inches diameter with a swivel head and long handle. You can see all the top braces, tail block, sides, etc.

I always feel the inside, loosen the strings, and put my hand in the hole, feel if there is any brace loose.
Than put a small flashlight in the hole in a dark closet, and inspect the wood for repairs, small line cracks, the light will shine trough.
Also take a small dentist mirror ( as sam mentioned)to look at the bottom side of the top ( you might find a signature or stamp there)