You have to run the engine to test them.
I have done a set on an old polaris motor, and when the motor came out, enough of the seal was missing, I could see a large chunk gone. Replaced the seals, and that old 340 is still running strong
The way to test them is while the engine is running, spray carb cleaner, or WD40 down behind the clutch, close to the shaft. If the motor revs, or changes pitch, then you have a bad seal on that end. Cannot really check the seal on the other end very easy I do not believe....