Go to the Pokemon cente in eterna and talk to the lady with brown hair and she will give you the happiness app (the one with all your Pokemon floating around and bumping into each other). Hold your stylus on apokemon of your choice and if 2 big hearts come out of it, its happiness is near if not full.
Chat with our AI personalities
Level it up when it its happiness is full
In goldenrod, by the bike shop there is a house. Talk to the lady inside. She will say something like: Your Pokemon is still warming up to you (low happiness) Its quite cute (kinda low happiness) Seems to like you a little (kinda high happiness) It really Trusts you (Max happiness)
in cerulean city in a house next to the PC get pikachu to full happiness and then talk to her
Level up an Eevee with full happiness at nighttime (after 6pm)
In salport city there is a woman looking at a picture in the Pokemon fanclub. Make sure the first Pokemon in your party has full happiness's. Then talk to her and she will give you the soothe bell.