Telltales

noun

1. One who divulges private information with intent to hurt others.
2. Tattletale; squealer.
3. An indicator, such as a warning light, that serves to warn of a hazard or problem.
4. Something that serves to reveal something else.

   "The telltale was the lipstick on his shirt collar."

5. A movable piece of ivory, lead, or other material, connected to the bellows of an organ, whose position indicates when the wind is exhausted.