Noun

1. one who divulges private information with intent to hurt others
2. tattletale; squealer (U.S. slang)
3. something that serves to reveal something else

   The telltale was the lipstick on his shirt collar.

4. 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

Adjective

1. revealing something not intended to be known

   His eye was blinking, a telltale signal that he was lying.

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