Is there an obvious test for this that a user can do?
Not that I am aware of. We had a sparky come in to look at the whole thing - it was only when doing a fuller inspection that he caught it. It didn't seem to affect anything day to day, but any surge would have killed all the things in our house!
Spooky. We had the same issue at ours - 10 year old CU installed fresh as part of an extension before we bought the place.