In Pokemon Pearl, How do you tell if a friend is connected on wi-fi?

Answer 1

When you go on wi-fi, you will see your friends online. If you don't see them, that means they are not on.

Answer 2

there is a poke app for just that once you obtain it you go to it and it will tell you who's in the : mine's , union room , wifi , and just anywhere you can play with someone through the game.