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.
Chat with our AI personalities
Kinda. Just battle with him a lot, and check his happiness with your Poketch app that shows you the hearts.
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.
check your national pokedex check serebii.net :)
talk to your first party Pokemon behind you.
In diamond, pearl, and platinum, there's a lady in the Eterna Pokemon centre who gives you the friendship checker poketch app, you tap a pokemon, and if no hearts appear, the pokemoon doesn't like you, if one heart, it's warming up to you, two hearts means it likes you, and two big hearts means it loves you, so if a Riolu, Eevee (if you want an Umbreon or Espeon), Sneasel, Happiny, etc., it must mean it is close to evolving.