In my experience, they fail in the unlocked position. The only way to test them, is to drive hard enough to start overheating. Then get out, take a newspaper or such and try to stop the fan. If it stops, the coupling is not working.
It is fairly hard to get into a condition ( in the UK) where the coupling locks up. It needs to be hot out and you need to be using power and moving slowly. A long steep hill when it is 30 C out is what you need.
Like I said, I have NEVER found a used LR with a working coupling. Most people can't tell because they never see the conditions where it is needed.