Use spectator to know for sure if anyone is online. But i think most people gave up on the wifi for Brawl because of lag and failing to connect with other players. Someone's firewall can easly prevent connecting with other players, which is very annoying because most people don't know the issue nor don't know how to fix it with port forewarding. Even the DNS server can prevent from connecting with other players, thats why i use the servers of OpenDNS.
Other thing i noticed with wifi is that if a player fails to connect because one of the other players has one of these issues, then it prevents them from connecting. It would have been way better if one player becomes the host and a host without issues is reliable, because that way everyone can connect. Capcom did that right with MH3, especialy when you can choose the server and rooms yourself.