Unknowingly providing a verbal or  physical cue. Most often used in reference to knowing when someone has a very good poker hand, some physical change in their appearance ‘tells’ other players they are in the lead. For example, if someone receives 4 aces in a poker game and they have a big grin, that would be a tell. (Compare to Give  yourself away.)

“I know when I’ve hurt her feelings because she has a tell,  her ears start to twitch.”