If you mean the app. then you will need to be at the begining when dawn tells you to choose what hand you want. You will need to choose the right-hand for the app.
In Pokemon Diamond the Friendship Checker (also called the Happiness Checker) app is #6.
Just go to Eterna city, and go into the Pokemon centre. Talk to some lady (i think she has green hair), and she give you the friendship checker app on your Poketch.
Talk to a woman in Eterna city Poke center and she'll give you it.
The 6th application, the friendship checker can be obtained from a woman in the Eterna City Pokemon centre
press your stylus over the Pokemon you wanna check your friendship with one heart - ok two hearts - good two hearts that grow - excellent
In the Eterna City Pokémon Center, a lady in there gives you the Friendship Checker App for your Pokétch. By touching the Pokémon of your choice, hearts will show up.
You can get the friendship checker in HeartGold at Goldenrod City. Talk to the happiness rater and she will say a phrase that indicates your Pokémon's happiness. This determines your friendship levels.
Get some mates.
Fly/walk to Eterna City and go into the Pokemon center. Talk to the girl inside there and she will give you the friendship checker.
in golderod city is a frienship rater
The friendship checker (my personal favorite app) is inside the Pokemon center of eterna city. Talk to the lady with brown hair right byt he tile pokeball in the floor.
220 or more, it should show 2 hearts on the friendship checker.