A friend of mine has some questions but his english is not good enough to ask. So I'm doing the job:

What can Nintendo detect when you go online?

IOSes installed?
IOSes modified (cIOS, Hashsums?)
Channels installed?

What can they detect when you play online (like Mario Kart)?

Nothing. They can't detect anything.

AFAIK, all they can see is your play history, and that is ONLY if you install the Nintendo Channel, and agree to send it to them, and even then, they claim it is completely anonymous.

Thank you guys :).

If you use cheats online and they have folks in there, they'll obviously be able to see that... that's where your bans come from.