Cats don’t talk because if they did, they wouldn’t want to say anything to people anyways. Since cats can’t verbally communicate, it’s up to you to decipher their cryptic body language to determine if they’re happy or not.
Generally cats are happy if they feel safe enough to play, flip over on their belly, or rub against you. When cats are happy, they can make you happy and when cats are mad, they’ll likely make you mad too — just like children. The only difference is that cats won’t grow up and become teenagers one day.