someone who gossips indiscreetly
"a telling smile"
"a telltale panel of lights"
"a telltale patch of oil on the water marked where the boat went down"
- The telltale smell of cigarettes told her that he had been in the room.
- I could see from the telltale look in her eyes that she loved him.
- The teacher did not notice that I was cheating until that rotten telltale told her.
- There are a number of telltale signs that an application is leaking memory.