They should be checked before and after every dive. The process is a simple visual inspection and you should look for signs of abrasion, an air bubble that is larger than 1/2 the volume, or any signs of contamination (dirt or milky looking oil inside). If any of these are found, the cartridge seal should be replaced.
It is easy to remember the inspection requirements if you think about the ABCs of cartridge seal inspections:
Abrasion
Bubble
Contamination