This is pretty much an unenforceable rule. Certainly you could say the 6-8" of wire exposed in the box is statistically likely to contain the once every 2' writing but it is not a certainty. I also have not seen anything embossed on the jacket on most of the MC I see. Maybe it is just my old tired eyes, I have a hard time reading glass fuses too.
I think we all ignore those device to device jumpers that are made up from scraps of wire from cables.
Certainly it is a rule and if I see something that looks like "automotive" wire I would get a bit more curious but if it looks like THHN and it is the right size, I would give them the benefit of the doubt and assume it is THHN. I guess you could chew on the insulation a little and see if it tastes like THHN.