There's a stretch limit, you count the pins (21 I think) then check the length between the 1st and 21st pin, should be less than 323mm to be within tolerance, I think, check the manual, I'm fairly sure it's 323 for a 525 chain.
A less scientific test is press the chain tight on the bottom run and try to pull the chain away from the rear sprocket, if you can see daylight through the gap between the chain and sprocket consider changing them.