The function of the timing belt/chain is to harmonize the engine’s camshaft, crankshaft, distributor (if vehicle has one) and the water pump.
A common sign of wear would be the engine stopping or the engine backfiring.
The function of the serpentine belt is to transfer power from the crankshaft to the alternator.
Common signs of belt wear include cracking, splitting, squealing, dimming headlights while driving, or loss of battery charge.
Inspect all drive belts every 15,000 miles/12 months. Adjust V-belt tension every 30,000 miles. Replace drive belts and timing belts every 60,000 miles or according to your owner’s manual for service recommendations.