(verb)
to communicate something to a person in speech or writing, such as to provide advise or instructions, narrate a story, or relate facts or information;
e.g. When you see your mother, tell her I said hello. This note will tell him what to do next. I promised to tell them a story before bed.
(verb)
to show some information without verbally stating it, or to reveal some confidential or private information;
e.g. These results tell me that the new product works better than the old one. I promised I wouldn't tell if she wouldn't
(verb)
to know something with certainty, to correctly determine something, or to distinguish the difference between two or more people or things;
e.g. She could tell that he didn't feel like talking about it. He struggled to tell the difference between the two products.
(noun)
a gesture or action, often done unconsciously, that is considered to betray someone's attempt to lie or deceive, especially in games like poker;
e.g. I could identify his tell, and it was that his eyebrow twitched when he was lying.