In Diamond, Pearl and Platinum, a Pokétch app can be obtained from a brown-haired girl in Eterna City's PokeCenter. This application displays the Pokemon in your party. When you pin them down with the stylus, a certain number of hearts will appear to show the level of happiness that your Pokemon have. A Pokemon that is near max happiness will have two large hearts.
Near max happiness = 250+ (out of 255)