If it has nipped up on one of the cylinders it will more than likely be obvious just from looking up the exhaust ports.
Use a torch and try to raise the front of the bike if you have it on a paddock stand. If you do it on the side stand you'll have to lay down on the floor.
Generally you will see the piston skirt will be scuffed and scored and maybe the top edge, (crown edge), eroded or chewed up slightly. There may be smears of piston material over the rings too.
If the skirt is very slightly polished or worn, it's probably not worth worrying about.
If you are not sure, take a picture if you can. If you can post it on this thread, someone should be able to guide you. You'll always get a lot of opinions on here!

As dunkenb said, another way will be to look at the spark plugs. If you see little silver spots/deposits of metal splattered over the plug, it's a good indicator of that plug's cylinder having a problem.