See if there is a person that will look at your Pokemon and tell its happiness if there isn't a person in the game that doesn that teach the Pokemon Return keep using it until when it uses the moves it almost doesn't stop moving that is when your Pokemon is the happiest.
Use items.
Max out it's happiness.
Pichu evolves by happiness.
For a togepi to evolve you need to give it 220 happiness
To see a Pokémon's happiness in HeartGold or SoulSilver, you take them to Goldenrod City, where there is a house where you can check your Pokémon's happiness there. It is on player's right-hand side, and is in the little section right below the Magnet Train.
Any level, just get it to max happiness.
Level it up at night whilst it has high happiness.
You can't find registeel in Pokemon HeartGold.
You can find it in Pokemon Platinum, Pokemon Crystal, Pokemon HeartGold, and Pokemon SoulSilver.
unkowns {pokemon}
You cannot find Latios in Pokemon HeartGold unless you went on wi-fi and got the Enigma Stone
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)