I don't know if there is any way to tell without just replacing the o ring first and seeing if that resolves the problem. I did mine (oil, mech seal, o ring, etc) this past summer and it does not leak, but I had the same thoughts as you when I was doing the work. Would have saved A LOT of time. For what it's worth, I was able to do all my work with the engine in the sled. I think most guys pull the whole thing. Good luck!