Go to Vendanturf town Talk to the lady in the house beside "Wanda's House"
Note: the Pokemon that you want to check it's happiness must be in front of the line.
Chat with our AI personalities
you can check a pokemons happiness in Diamond/pearl by the use of a poketch app, in previous games you could only know in the happiness was maxed by obtain a ribbon from a certain character depending on the game.
get the Poke tech App Poke happyness
Check the TV every day.
There's a Poketch application which you can use to check your Pokemons feelings for you. You can get it at Eterna City's Pokémon Center, if you talk to a brown-haired girl, who'll give it to you. Also, there's a woman in Heathrome City who can tell you how your Pokémon feels about you. I think she's located in the Pokemon Trainer Fanclub.
To check a Pokemon's friendship status (how much it likes you) you need to get pokétch app # 6. when you have this app open on your bottom screen, press and hold the touch screen where an icon of one of your Pokemon is. when you touch its icon, you get one of three reactions: nothing happens, small hearts appear, or big hearts appear. Nothing means the Pokemon is unhappy with you and doesn't like you yet. Small hearts mean the Pokemon somewhat likes you. Big hearts means it really likes you.As for evolving by happiness, you usually need it to have big hearts. I have only had one instance in which my Pokemon happiness evolved with small hearts.