1. You could try a leak down test at 7-10 psi, spray some soapy water on it and see if bubbles come out, if it does you should probably replace.
2. Also, you can pull the inspection plug on the stator cover, cover the plug with your thumb or something. Fire it up for a bit, shut it down, release the hole and if you hear air being drawn in/vacuum, the seal probably needs to be replaced - a trick Art taught me. I am not real experienced at this method, just wisdom I have heard. The pto side has two seals so its not as likely to leak.
I have also heard OEM seals are the only way to go from other sources, if you need to replace. Just what I read when researching cranks, not sure if it's true or not.