Instructions
Defeat Gym Leader Falkner in the first gym in Violet City. You will be awarded the Zephyr Badge.
Go to the basement of any Pokemon Center. Speak to a woman named Teala. Teala will give you the Pal Pad so long as you have the Zephyr Badge in your inventory.
Open the "Key Items" menu and select the Pal Pad. You can now check all of your registered friends.