With cats and dogs it’s all about the tail from what I’ve observed. They can’t really smile but their wag (dogs) or tail position (cats) convey their feelings. If your cat’s tail sticks straight up when they see you you know they have an affection towards you. Or maybe it’s food. All I know is straight up tail is a happy cat.

If it’s swishing, look out. And a dogs tail stops wagging and goes into like a prone state when they are concerned.