in diamond get a poketch app from the woman in the eternal city pokemon center. check it and the pokemon in it is happy
In Fire Red and Leaf Green you need to go to Gary's home (your rival) and talk to his sister. She will tell you if your Pokemon is happy.
In Gold and Silver (they are getting remakes soon called Heartgold and Soulsilver) you need to go to a house in Goldenrod City, close to the bike shop. The girl there will tell you how happy your Pokemon is.