While Pokemon Legends: Arceus makes many substantial changes to the traditional Pokemon formula, both in exploration and combat, there are still lots of mechanics and features that remain exactly the same. Like in previous Pokemon games, players can check their Pokemon's friendship levels in Pokemon Legends: Arceus. As to why a player would need to check if a Pokemon is happy or not, it's because there are specific Pokemon that can only evolve by increasing its happiness levels to a certain amount. As for the Pokemon that evolve through high friendship/happiness levels, some examples are Riolu and Chansey, which can evolve into Lucario and Blissey, respectively.
Before players unlock the Friendship Checker in Pokemon Legends: Arceus, they will need to have made some progress through the main story Missions. These are different from Requests, as completing them moves the plot of Pokemon Legends: Arceus forward. Players need to complete Mission 7 "The Frenzy of the Lord of the Woods," and then start Mission 8, called "Arezu's Predicament." After getting to this point, players should be able to find an NPC named Belamy hanging out near the Pastures in the southeastern corner of Jubilife Village.
Complete Belamy's Request
After speaking to Belamy for the first time, he'll give players a simple Request to complete, and it's called Measuring Your Compatibility (Request 28). After completing the quest by showing him a Pokemon with high friendship, players will finally unlock the Friendship Checker in Pokemon Legends: Arceus.
How to Check Friendship Levels in Pokemon Legends: Arceus
The Friendship Checker isn't an item or an application that can be used like in Pokemon Brilliant Diamond and Shining Pearl. Instead, Pokemon Legends: Arceus players will need to come back to this area and speak to Belamy for him to check the friendship/happiness levels of a Pokemon.
Players can choose one Pokemon at a time from either their party or one of the Pasture boxes. Belamy will give players a short description of how the Pokemon feels towards the trainer, so now players will know if the Pokemon needs more attention before it evolves into a more powerful Pokemon. Players can increase a Pokemon's happiness by leveling it up in battle, or by using Rare Candy. Spending time with a Pokemon, in general, can help increase its mood, and make sure that it doesn't faint often in battle.
Pokemon Legends: Arceus is available for the Nintendo Switch.