Specifically, if there is a vent hole in the foredeck, inspect all around it for rot in the plywood core.
Walk on the foredeck-- does it feel strong like a table, or bouncy or flexing.
Cal 20s are famous for breaking the hinges where the spreaders connect to the mast. You can buy new, heavy duty ones to replace the originals. See how much of the standing rigging is original.
What is the condition of the rudder-- wood or fiberglass, solid or water logged.
Keel bolts-- new and stainless, or rusted, swollen and leaking. FYI, the bolts on a Cal 20 are available new and very easy to replace-- they pass through the flange of the keel, but they are not threaded into it.
You can see our restoration sequence of a Cal 20 here--
Cal 20 Restoration
Cal 20s are simply the optimal starter boat for many people. I much prefer them to trailer boats, and the one design aspect have kept good sails and parts available for them. Affordable, but fun and safe to sail.