I suspect C3's are similar to C2's so check the A pillars rust bubbles, particularly the lower section, the door jambs, door strikers, windscreen frame - see if you can remove trim panels, bird cage - ask if you can get them to remove the glove box.
Get a camera and take photos of underneath the dash then view them, those probe cameras come in handy.
Remove kick panels near where your feet go. Inspect the frame underneath - good if you can stab it with a screwdriver to check but some may not like everyone doing this. Take a look at the suspension, you don't want it heavy in rust.
Rub your hand along the wheel wells and check for cracks.
Check the VIN but this is like numbers matching and if you want original options, non matching is a deal breaker to some.
s