telltale

1. One who divulges private information with intent to hurt others.
2. An indicator, such as a warning light, that serves to warn of a hazard or problem.
3. Something that serves to reveal something else. (figuratively) · The telltale was the lipstick on his shirt collar.
4. A movable piece of ivory, lead, or other material, connected to the bellows of an organ, whose position indicates when the wind is exhausted.
5. A length of yarn or ribbon attached to a sail or shroud etc to indicate the direction of the flow of the air relative to the boat.
6. A mechanical attachment to the steering wheel, which, in the absence of a tiller, shows the position of the helm.
7. A compass in the cabin of a vessel, usually placed where the captain can see it at all hours, and thus inform himself of the vessel's course.
8. A machine or contrivance for indicating or recording something, particularly for keeping a check upon employees (factory hands, watchmen, drivers, etc.) by revealing to their employers what they have done or omitted.

Plural telltales