The easiest way to tell if your gas rings are in need of replacement is if you extend the bolt out of the carrier as far as it will go and place it like so on a table.
If the carrier falls down over the bolt under its own weight like this, the rings are worn out and need to be replaced. To replace them simply take the carrier apart as described in the Rifle Disassembly, Cleaning and Maintenance section of your owner’s manual. Use the tip of a razor knife to pluck the rings out of the slot in the bolt, and just chase them around with the blade until they all pop out.
Then take your new rings and with your fingers chase them around the slot until they are all in place. Separate the breaks in the rings so they are not all in line.
With the new rings in place the carrier should no longer fall over the bolt under its own weight and you are good for several hundred more rounds.