The pulley is made from an outer section and an inner section, and the two sections are joined by a strip of rubber about an eighth of an inch thick. The entire pulley assembly is often called the harmonic balancer, so named because the rubber joint dampens some of the harmonic (rhythmic) vibrations inherent in the engine.
Try to keep a permanent line marked across the rubber joint of the crankshaft pulley. Example pic. Clean the pulley off thoroughly, then paint your line or lines. Neatness doesn't really count all that much.
The reference line or lines will make it easy to spot any slippage between the two parts of the pulley.
If you notice any slippage, replace the pulley ASAP, before the joint fails completely. If it falls apart, it can be destructive and you will be left stranded. Any time you hear any strange squealing noises on startup, check the pulley ASAP.