telltale

One who divulges private information with intent to hurt others.

Tattletale; squealer.

An indicator, such as a warning light, that serves to warn of a hazard or problem.

Something that serves to reveal something else.

“The telltale was the lipstick on his shirt collar.”

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

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.

A mechanical attachment to the steering wheel, which, in the absence of a tiller, shows the position of the helm.

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.

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.

A bird, the tattler.

A story or fable that has a moral or message.

Revealing something, especially something not intended to be known.

“He blushed when he approached, a telltale sign that he was happy to see him.”