You should inspect your serpentine belt periodically, say every time you change the oil or add windshield washer fluid to the reservoir.
If the belt is badly cracked, frayed or has chunks of rubber missing from its ribs, the belt should be replaced immediately.
Minor cracking on the ribbed side of the belt is okay, but heavy cracking would tell you the rubber is getting hard and the belt is near the end of its service life. But this only applies to older serpentine belts made of neoprene synthetic rubber.
A good rule of thumb for older chloroprene-based serpentine belts (neoprene belts) is that if cracks are observed 3 mm (1/8 inch) apart all around the belt, the belt is reaching the end of its service life and should be changed. Small cracks spaced at greater intervals are normal on older belts as they age, and do not mean the belt needs to be changed.
With late model serpentine belts of EDPM, you probably won’t see any cracks because EDPM seldom cracks as it ages. This makes it more difficult to determine the condition of the belt. A high mileage belt may still look like new, but the V-grooves on the underside may be badly worn allowing the belt to slip. Slippage can lead to noise, pulley wear, engine overheating, battery undercharging, and even false engine codes (A/C compressor slippage codes, power steering codes, knock sensor codes, etc.).
A classic symptom of a worn serpentine belt is belt noise when you rev the engine. If the ribs on the underside of the belt are worn, the belt can lose its grip and slip under load. A weak automatic belt tensioner or insufficient tension with a manually adjusted belt can also allow the belt to slip. If you see any belt flutter when you rev the engine, it probably does not have enough tension.