A quick check is the lift check. Wrap the chain around it through 180 degrees, as if in use, and lift the link that would be at 9 o'clock if it was maounted. Pull the link forward. If you can pull it forward enough to see the bottom of the roller trough in the sprocket any at all, it's shot.
I don't have to measure or pull anything to tell you the one in post #2 is shot to shit. It is also correct that you should be replacing the chain and the rear sprocket as well. They wear as a set, they get repalced as a set.