The upper o-ring is easy enough, just pull the valve covers and fire it up- watching for any oil coming from the base of an injector. The lower o-rings are a little tricky, since you cannot physically see them. Symptoms would include excessive oil consumption, bluish smoke, and a black fuel filter. If it is more than just a little, observing a higher than normal IPR duty% would happen, and if it's real bad, you get an uncontrollable IPR code. As for picking exactly which one is bad, it really doesn't pay to try and do just one. You would be much better off doing all 8 since it's cheap, as far as parts for these things, but a pain in the keester. If you go ahead with it, make sure to read the posts on how to go about doing it and what to expect when trying to start it back up for the first time.