First, choose which pokemon you to give max happiness to, through the [Pokemon] option in your menu. Click your [A] Button while on that pokemon, and it will bring you up a new menu, with the first option being “Summary”. Now, hold down [L Trigger + R Trigger], and hit the [A] button on your AR, and look at your Pokemon’s stats. You can’t see it, but your pokemon now has 255 happiness. Use the above code, the Pokemon Data Checker, to verify your happiness.