Yearly I do the lighting to the local HS musical. As part of that, I fix whatever broken lights I can and do basic maintenance. One of the issues we've repeatedly run into is lamp sockets going bad. I'd like to mark the fixture caps as to if/when I've replaced the socket. Marking the yoke won't do anything as the caps get swapped around from unused fixtures to broke fixtures being used. In the past, I've put colored electric tape on the whip, but that doesn't say when it was changed, just that it was changed.