How do you know if your Pokemon is happy or not?

IN GBA GAMES, to tell if it is happy, use an item on it like potion or rare candy. in the short scene after you use it, it will scoot forward, backward, or stand still. if it scoots forward, it likes you. If it goes back, it dislikes you. If it stands still, it's neutral.

IN DIAMOND AND PEARL, talk to the footprint guy.