Useful guide here
, pretty much the same as you'd look for in any second-hand car.
The most important being the paperwork, proof of cambelt change, electrics and what it sounds like. The weakest part of the GT/156/147 is without doubt the suspension, listen for knocks and look at the tyre wear, having said that even if it's OK now prepare yourself for problems at any time.
Check the exterior door handles operate smoothly, these have a habit of breaking at the hinges.
Look at the rear edge of the bonnet, the driver's side wiper arm tends to catch and wear the paint away and wear a groove in the wiper arm.
Cosemetic stuff like stonechips, alloy kerbing, seat wear, interior door grab-handle wear are inevitable, only you can decide whether it's acceptable.
All the best