First, check your oil level with the bike vertical. If you're sucking oil up from the crankcase it'll show in your air box.
Check/clean or replace your plugs.
Now if you really think your rings might be out, check the compression. Some auto parts stores have the kit to rent. do it once, then squirt some oil into the cylinder and do it again. If the compression changes, it's the rings. If it doesn't, its the valves/head. If you don't know where your compression should be, just look for a cylinder that's about 5lbs or so different than the others.