telltale

noun

a person who tells secrets or informs; talebearer; tattler
an outward indication of something secret
any of various devices for indicating or recording information; indicator; specif.,
1. ☆ a row of dangling ropes, etc. hung over a railroad track to warn trainmen, riding on top of a car, of an approaching low bridge, tunnel, etc.
2. a device indicating the position of a ship's rudder
3. a time clock
4. a gauge on a pipe organ showing the air pressure

adjective

revealing or indicating that which is hidden or meant to be secret: the telltale smell of alcohol on his breath

See telltale in American Heritage Dictionary 4