telltale
n.— «The replacement part turns on a little telltale next to the odometer display indicating that the part has been replaced.» —“Re: reprogramming digital odometers” by Mike Griese (magriese@polaris.rchland.ibm.com) Usenet: rec.autos.marketplace Apr. 7, 1995. (source: Double-Tongued Dictionary)
Further reading
What if, instead of being an inanimate object, a dictionary were alive? That’s the idea behind a lavishly illustrated new children’s book called The Dictionary Story (Bookshop|Amazon) by Oliver Jeffers and Sam Winston. This is part of a...