How do you know someone is happy. Well, typically, a happy person is smiling and being friendly and all that nice stuff; however, if you happen to eat the very prop this person was using to demonstrate what it's like to be happy, he may become mad. How do you know if someone is mad. They'll usually be yelling or threatning to tell your mommy about what you did. Find out whether Kermit is happy or mad in this video. Subscribe to the Sesame Street Channel here:.