How To Check Friendship Level - Pokémon (BDSP)

First you need to go to Eterna City.

Click Town Map option.

Enter in the Pokémon center and have a talk with NPC here.

You obtained the friendship checker app.

The friendship checker app identifies the Pokémon that like you.

Open the app and click on the Pokémon to see the friendship level you have with them.

More hearts means better friendship.