Measure the chain stretch, I believe 12.71 inches over 20 links is the service limit. Let me double check...
Okay the service limit is 12.72 inches (323mm)
over 20 links with 20 pounds (10kg, 98 N) hanging on the bottom of the chain to keep it tense.
Measure it in multiple spots though, in case the chain isn't stretching uniformly.
Sources: My service manual