You need to place the DS stylus over the correct Pokemon and if nothing shows up then the currently selected Pokemon does not like you. However, if one or two hearts are shown then the Pokemon does indeed like you, with one hearts being less liked than two hearts.
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In Pokemon Diamond the Friendship Checker (also called the Happiness Checker) app is #6.
The friendship app or frenship checker is given to you by a girl on the first floor of the Eterna city Pokemon center.
There should a guy inside the Day Care building, sitting at a table and he'll give you the Friendship App if he's there when you talk to him.
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.
You can find the app in the Pokemon Center in Eterna City. Talk to one of the girls.