Talk to a woman in Eterna city Poke center and she'll give you it.
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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.
press your stylus over the Pokemon you wanna check your friendship with one heart - ok two hearts - good two hearts that grow - excellent
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 goldenrod on the right side of the town