Check that the cambelt etc has been replaced in the service history, or budget for this when negotiating a price.
Check the MOT has a decent amount of time left on it.
Suspension coil springs if standard are prone to snapping, which would mean an MOT failure but can also be hard to spot if its at one end. Try to jack it up and check that way.
Check the oil cap for residue, if the cars been used for lots of short journeys, or used around town then some residue is common. If its excessive, walk away - same goes with the dipstick.
Start the car from cold, idle should settle and be smooth after about five minutes. Check for smoke from the exhaust. As it's cold, smoke will no doubt be visible just make sure its not excessive and not blue, get someone to rev the engine while you stand behind it to check that no oil seals (or worse) have failed...
Check all your lights, wipers, switches and that the air con works (if fitted).
Check the model has everything it is supposed to have, if its a Gordini it should have stripes, badges and logo'd floor mats (if spec'd). Same goes with other special editions.
But in general, the Twingo RS is a well built and reliable car without any common big problems. Do you have a link to the advert of the car your going to view? Or some pictures? So we can have a quick look and make sure there's nothing that might be an issue (wrong wheels etc).