Native tethering only allows you to use USB/bluetooth. iPad requires wi-fi. If you want to use wifi then you need to JB and use PDANet or Mywi.
My carrier (HK vodafone) definitely can tell if you are using your phone as modem. I was charged for $100USD for my data (tethering) even i had unlimited data plane. I believe all carrier can tell if you are tethering or not but whether or not they do anything is a different story. Almost all people said ATT won't do anything but in my case, i was charged.