Overheating - Check the car for any signs of overheating.  If there is consistent over flow from the expansion tank or signs of evaporation on different parts of the radiator then it is a good bet that the cooling system has deep problems.  
(Deduct 30 points for chronic over heating problems)

Radiator - Check for leaks around the seams - the radiator is aluminum and has plastic end tanks.  The end tanks can separate, crack or be damaged by stones from the road.  A tell tale sign is a white corrosion along any of the seams.  
(Deduct 4 points for a cracked end tank)

Original Stickers - Check for original stickers.  There is generally three gold anodized stickers on the top of the radiator.  These are hard to come by so can be valuable for restoration purposes.  It is also an indication that the car is close to original.
(Deduct 4 points for a cracked end tank)  

Radiator Core Repair - The headlight motor is mounted in front of the radiator.  It is not uncommon for this bar to either fall off or during repairs on the headlights to puncture the radiator.  Look for repairs in this area and since the radiator can be flipped 180 degrees check the same area under the fan. 
(Deduct 15 points for a bad core radiator) 

Custom Cooling Components - DEVEK makes a high capacity all aluminum radiator that is a valuable addition to the 928.  There are also a few companies that make conversion electric cooling fans.
(Add 10 points for a DEVEK all aluminum radiator.)

Water Pump and Thermostat  The water pump is hidden by the engine covers so it is not possible to check for end play or even see any possible leaks.  However, the top part of the pump is visible just behind the fan mount or the plate that holds the wiring harness on the 1987 and up.  When the pump is rebuilt it is generally bead blasted which gives this portion of the engine a clean and distinctive look.  Over time the top of the pump will become dull and oxidized.  The water pump should be changed along with the timing belt every 30 to 40K miles or five years.  Age like the rest of the cooling system is critical.  
(Deduct up to 15 points for the age of the water pump prorated for  over 10,000 miles and 2 years)

Hoses - Probably one of the biggest areas of failure are the cooling hoses.  The lower radiator hose should be secured to the radiator shroud by a tie strip.  On the pre 1985 this was critical as the lower hose can be cut by the power steering pulley and fail.  Look for scrape marks in this area.  The lack of a tie strip is a good indication of the care and quality of mechanical work performed on the car.  Check all hoses for flexibility.  There are several small hoses running from the top of the radiator to the expansion tank and heater hoses on the rear of the engine that are often over looked.  Most of the hoses on the car including the overflow hose for the top of the radiator to the expansion tank are formed hoses that are available only from Porsche.  Due to the close tolerances in the engine compartment is difficult to substitute standard radiator hose and not restrict water flow. 
(Deduct 6 points if the all of the hoses have not been replaced in the past three years.  Deduct an additional 2 points if the hoses are not all factory hoses. Deduct 1 point for each individual bad hose up to 10 points)

Fans Pre 1987- The pre 1987 models had a viscous cooling fan that contains silicon oil.  Over time the silicon oil will leak out and the fan will not push enough air to cool the engine.  The fan can be refilled but  there is no approved way to test it.  Check the fan by spinning it by hand and looking for consistent resistance through the entire 360 degree rotation.  Also check the fan for play as the bearing can wear.  If a fan comes off at speed it can do considerable damage. 
(Deduct 10 points for a bad fan or clutch)  

Fans 1987 and up  In 1987 Porsche replaced the mechanical fan with dual electric fans.  These should come on when the car gets hot run for a few minutes and then shut off.  Fans that run all the time are indicative of bad sensors or at worst a bad cooling system or even deeper engine trouble like a blown head gasket or cracked block.  The fans should spin freely with no vibration or noise.  (Deduct 10 points for each bad fan and 5 additional point if the electric fans run all the time.)

Flaps  The S4 model after 1987 added flaps to the front of the car.  The flaps are designed to open when the engine gets up to operating temperature.  In cold climates the fans serve the purpose of warming the engine up quicker avoiding ring wear due to wash from raw gas.  In many cases the flaps are disconnected by the owner and remain open at all times.  It is important to insure that the flaps do not stay shut once the car is at operating temperature.  
(Deduct 2 points for non opening flaps in warm climates and 5 points for non operational flaps in cold climates)

Expansion Tank  The expansion tank lives on the right side of the car and is part of the pressure system.  The radiator cap should be in good working condition and the tank clean.  It is common for the tanks to fail after a number of years so discoloration is important to notice.  A new tank is almost white and can be seen through easily.  There is a water level sensor built into the tank.  The wires going to it can become corroded and the boot can protective boot can deteriorate.  
(Deduct 2 points for a bad or discolored tank)

Cooling Pipes - The expansion tank is connected to the engine by a steel pipe that tees into the engine.  Because this pipe is steel the ends can deteriorate or in the extreme the pipe can develop pin holes.  
(Deduct 2 points for a rusted connecting pipe)

Heat Riser - On the earlier models the radiator water was used to heat the breather tube running from the oil filler to the air cleaner.  This is fed by two hoses - one from the back of the thermostat housing and one from the rear of the left head.  The tube running thru the breather can become clogged or leak.  It is not uncommon for the feeds to this tube are blocked off.   
(Deduct 2 points for a disconnected heat riser tube)

Coolant - The aluminum system is sensitive to corrosion and should always be run with a mix of 50 / 50 anti freeze and distilled water.  The new silicate and phosphate free coolants are a must for the longevity of aluminum parts.  These antifreezes are colored orange.  The coolant should be renewed every 5 years or 15,000 miles. 
(Deduct 1 point for regular yellow/green antifreeze or old antifreeze - deduct 5 points for water only or if rust can be seen in the system)

Shrouding - The radiator is installed on rubber mounts and has a two piece plastic shroud around the fan on the older models. There are also tow side shields that keep air from leaking around the system.  All these should be in good condition.   
(Deduct 2 points for missing minor parts)