I agree with all that has been said.
If you have external cables on your bike, you can shift to the largest cog (ring usually refers to the front, not rear cogs), then while pulling on the cable shift down all the way with the right shifter. You can then pull the cable out of one of the cable stops and push it so you can see the last several inches of cable coming out of the shifter (as you do when changing cables, but don't disconnect from the derailleur). If there is any fraying, time to change the cable.
It takes less time to do than to explain.