I don't believe I can instruct you how to hack your Wii because of the rules, unfortunately (I certainly do know how). =(
But I can definitely tell you the effects hacking has on online games. When someone is "hacking" online, this is generally referring to someone illegally using cheat codes, through hardware or software not endorsed by Nintendo. "God mode" is probably the most common in any online game, which as it sounds, makes you impervious to everything. Immature brats get a kick out of ****ing up everyone's good time by winning games this way, essentially.
Hacking cheats goes far beyond being invincible in a game, though. It can be a lot more devious and permanent. Like, for example, "hacked lobbies" present in almost every Call of Duty game. I don't know the logistic details, but people will create cheat codes to make anyone who enters their lobby, including themselves of course, to gain unbelievably huge amounts of Exp. Leveling up without working for it, basically.
To be honest, I wouldn't complain about a random boost in levels, considering leveling up itself isn't the joy of the game. Playing it is.
It's quite easy to tell if someone is hacking online once you've got a good eye for all the goings-on of a game. Eventually you'll know exactly what's normal and right, and experience can tell you when something's seriously amiss. But unless you spot it yourself, or another player confirms it, it's not possible to know if someone's hacking.