telltale
n.— «They’ve got a “tell-tale” tracking needle on the tachometer. Shows you the highest RPM a driver just reached on the track.» —“Studdard’s Ambition Burns On: At Age 51, SCCA Driver Wants to Try NASCAR” by Bill Robinson Atlanta Journal-Constitution (Ga.) Oct. 15, 1989. (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...