As Paul has said, there is no substitute for research, but this will at least give you a starter for ten:-
Bodywork - usual used car rules apply, look for even panel gaps (they should be even though not necessarily tight), signs of repair work and corrosion especially the sills, bottom edge of the front wings and around the rear wheel arches. Give the sills a knock with your knuckle and listen for changes in the sound along their length which might indicate dodgy filler repairs, give the under sill and wing plastic trims a firm wobble - especially the under-wing ones. They should be firm. If they wobble then it could be a sign that the fixing lugs and studs have corroded away. Open the front doors and remove the plastic vent you can see on the rear of the door frame. You can see down into the rear of the sill cavity. If there are signs of corrosion there then assume it is worse further down the sills. Other than that just use your common sense and look for the usual signs which might indicate bodywork problems or dodgy repairs. Finally check under the carpet in the boot, there should be a white sticky label with dot matrix characters on it. This is the car spec off the production line and lists the options, chassis number and other info. If it is not there it could mean the rear panel has been replaced after a rear shunt so query it or look through cars history for invoices from bodyshops. If it is there check the chassis number matches the chassis number on the bulkhead.
Look into the rear drivers side wheel arch and try to locate two metal pipes. These are the fuel pipes. If they look corroded/heavily pitted then they will most likely fail an MOT. Replacement with OEM pipes is very expensive but you can replace with flexi's which are much cheaper. This is becoming more common due to the age these cars are not reaching. Obviously not necessarily a show stopper but you need to go into this with your eyes wide open.
Mechanics - Not alot you can tell here unless you take it round to a specialists and pay for a proper inspection. Check for excessive smoke out of the exhaust, any strange noises coming from the engine, signs of mods. The rear transmission can make a noise like a bearing is going. You can usually hear it when ticking over in neutral and it quietens down/dissapears when you depress the clutch. This is fairly common and usually appears at quite low mileage and isn't necessarily a sign that the gearbox is shot so don't be alarmed. Also the gearbox can sound a bit whiney when driving, so again don't be too alarmed. However theengine should run smooth and pull well once the boost comes in.
NUMBER 1 GOLDEN RULE. CHECK FOR THE LAST TIMING BELT REPLACEMENT. SHOULD HAVE BEEN WITHIN LAST 40KMILES OR 4YRS. IF NO HISTORY OR IF OUTSIDE OF THIS THEN IF YOU ARE GOING TO BUY THE CAR CHANGE IT ASAP.
Obviously check for all the usual service/maintenance history. On old cars like these it may not be 100% complete but there should be enough recent history to give you the confidence the car has been looked after by the last owner or two.
Electrics - check they all work. They can be a real pain to track down problems and very expensive if you pay to have them repaired. A/C probably wont work and can be expensive to repair - however they tend to need charging every year so if the seller tries it on and says that is only needs filling then get him to fill it. It costs around £50 for a fill and if he's serious about selling the car he'll pay for that to sell a fully functioning car and prove it works. If he wont agree to that assume he's lying and then assume the car is a dog and walk away. You don't want a pup of a turbo on your hands. It will ruin you. If he's honest and says it doesn't work then its upto you assuming the rest of the car is OK, but obviously take that into account if you make an offer on the car. You never know it might be a cheap fix.
Clutch - these are quite heavy anyway compared with modern cars but they do get heavier as they wear. Also they can feel 'notchy' at the biting point when they get heavily worn. This is where the rivets fixing the friction material starts to bite into the driven plate. It's difficult to assess whether or not the weight of the clutch is heavy or not if you're not familiar with 944's already. A clutch replacement is circa 10Mh and £500 for parts so not cheap.
I think i'll stop there or this will turn into a thesis, but I think i've covered most of the main points. At the end of the day treat this as you would any other 20yr old car - with great caution. Also bank on having to spend £1k - £1.5k on various odds and sods in your first year.
Good luck.
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