Instructions
Travel to Goldenrod City.
Walk north from the Pokemon Center and turn east at the first intersection.
Turn right into an alley between two buildings. Continue south to reach a small house tucked away behind the Goldenrod Department Store.
Enter the house and walk up to the girl inside of it. Press the "A" button to talk to her. She'll offer to tell you how much the Pokemon in the first position of your traveling party likes you.
Press "Yes" to accept her offer. Her response --- ranging from "Your Pokemon doesn't like you at all" to "It must love you a lot " --- will give you an idea how the Pokemon feels about you.
Press the "X" button then choose the "Pokemon" command to switch a different Pokemon to the first position in your party.
Press "A" to ask the girl to analyze this next Pokemon. Repeat steps 6 and 7 for any other Pokemon with you so you know how they all feel about you.