I think there's way too much emphasis on attempting to detect the TVS failure. A shorted TVS (the most common failure mode) will be fairly obvious. An open TVS is difficult to impossible to detect in circuit, so that's a somewhat moot point. Also, the TVS parts really don't fail that often, standing on your head to detect the occasional failure doesn't make much sense. There are tons of other failures of electronics that are as frequent, but we don't attempt to have indicators for all of them.
I suggest you use protection devices in a proper manner and deal with the failures as they happen. 
