I've had a ton of fun with mine over the past year since I bought it. If you can tap around on the top of the deck, with something like the end of a wood hammer handle, you should be able to hear a reasonance difference in the areas of the deck that might have rot. Good deck should have a fairly sharp clean ping sound, areas with rot below should sound like a muffled, more dull thud like sound. All that being said, I believe it can be hard to tell, because where the fiberglass has a corner, or contour, it can still sound pretty sharp and pingy, just because of the way it's laid up.
Please take all this with a grain of salt, because I am definitely no expert, at all, on these matters, but that's the way I try to judge it. If there is any water intrusion in the deck though, for a decent period of time, it's a fair chance there might be an issue in some stage of development. I had some rot under my starboard side stern deck cleat, and it sounded fine, and seemed completely sturdy and solid as far as the deck integrity. I actually think it was, and the fiberglass decking laminate seems pretty dang thick. The plywood laminate was actually only 1/4" and I get the slight impression it's biggest function was to help form the deck during layup. I'm not sure if all the decking, and side decking, is only 1/4" plywood though. C22's seem to be very solid boats, and I've been continually impressed with how solid they are when I'm taking something apart for some reason. I actually live in MD as well, and I got pretty lucky finding a decent one for sale the first day I looked. I drove to Ohio for it, but it was not a bad drive, and I'm definitely glad I bought it. People on this board are very helpful too-
