On the Cals, you'll want to take a look for rot where the chainplates bolt to the bulkheads. Also, the Cals have a steel "transverse" beam under the cabin sole pan where the mast compression post sits. You'll want to check that for corrosion if you can. It's not easy to do. If the limber holes beneath the sole pan clog up, then water can sit against that steel beam and it'll corrode and mast support will be compromised.
Other than that, you'll want to look for the usual wet decks and blisters. The Cals didn't use solid glass where hardware was through-bolted like on modern well-built Hunters and Catalinas, so definitely check for spongy decks, especially on the coachroof around the winches, cleats, and clutches and near the mast step.
Cals in general had a pretty good build reputation, but that doesn't necessarily mean anything on a given boat after 30 or so years. I had a Cal 3-29 that ended up being a project boat for a nice old guy. My point is that you should look an old boat over really well and not rely too much on its builder's reputation from back then.