You definitely want to check the "birdcage" (windsheild frame and t-top crossbrace) for rust. Depending on the year, you will also want to check floorboards for rust. Other than that, the typical used car check should suffice. Look underneath at the frame, bushings, brackets and such. You are going to want to drive it and check out the brakes. C3's are notorious for having leaking calipers. Make sure the headlights go up and down with no problem. Check the HVAC functions. Ask about the fuel gauge and whether or not it works. Check to see if the clock is working(replacements are expensive).
Other than that, Anniecorvette makes a very good point. It is all about what you are looking for. Are you mechanically inclined? Are you looking for something to work on in the garage on weekends? Are you just looking for something to get in and drive right away? What is your price range? That could be a huge factor in what quality of car you get.
Good Luck:cheers: