Similarly to your car, conveyor drive systems need regular maintenance and upkeep to run smoothly for a long time.
One of the main indicators of a worn chain is the wear elongation. If the chain has elongated more than 3% from its original pitch length, then it is necessary to replace the chain. This length can be determined by measuring from one chain pin to the next in a taut part of the chain (a slack segment of chain will not give accurate measurements). Using multiple pitches will increase the accuracy of your measurement. Look up the ANSI standard pitch for your chain (it may be stamped on the side bars) and compare. Chain wear scales are extremely helpful for this measurement.
A tell-tale sign that a chain is worn and elongated is that the chain will be riding high (or up near the tip of the sprocket teeth) on your largest sprockets in the system. If you see this, replace your chain immediately.
If your chain is visibly damaged, bent or rusty it is recommended that you replace it immediately.