get in there with a screw driver and start poking around. Typically, a glass boat will have a massive piece of plywood back there. It should be exposed in some areas for you to see. If there are wood planks, check to make sure they are not all spongy. Some boat builders completely cover the transom wood in glass, this is good because it keeps the moisture out. Sometimes you can remove a large bolt that goes through the transom (like a engine mount) to see the internal condition of the transom. Makes dang sure you reseal the bolt when you put it back!
Aluminum boats can often have wood in the transom too. Always look at this when you buy a boat of any kind. Glass boats are nice, I would take a glass over an aluminum any day for a bunch of reasons.