telltale

Short definition from Wiktionary

Noun

1. 1
   One who divulges private information with intent to hurt others.
2. 2
   An indicator, such as a warning light, that serves to warn of a hazard or problem.
3. 3
   Something that serves to reveal something else.
4. 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. 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. 6
   A mechanical attachment to the steering wheel, which, in the absence of a tiller, shows the position of the helm.
7. 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. 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.
9. 9
   A bird, the tattler.
10. 10
    A story or fable that has a moral or message.

Adjective

1. 1
   Revealing something, especially something not intended to be known.