The following contains factory instructions for pressure testing the oil cooler element used on normally aspirated 944s. If you have experienced a mixing of oil and water on your car and are replacing the oil cooler seals as a result, it's a good idea to pressure test the oil cooler element at the same time.
If you don't want to bother with setting up the test rig for the oil cooler element, simply take it to a radiator shop and they will be able to pressure test it for you.
Dust Cap (23 mm or 1 inch)
Hose Clamp for Dust Cap
Water Container to Submerge Oil Cooler
Hose large enough to connect to other Oil Cooler Nozzle (1 inch)
Adapter to connect air supply to 1 inch hose
Place a dust cap with clamp on one of the oil cooler nozzles.
Connect a 1 inch hose to the other oil cooler nozzle and, using adapters, connect and air supply to the 1 inch hose.
Submerge the oil cooler in a tank of water.
Pressurize the oil cooler, using the compressed air supply, to approximately 0.8 bar (12 psi).
Look for evidence of air bubbles coming from the oil cooler. If bubbles are present, replace the cooler element.