So I have AT&T with an unlimited data plan.

I am told by AT&T that if I want to tether my phone to my computer that I need to have the data plan that incluides tethering.. 59.99/mnth. I currently have unlimited data that does not include tethering at 29.99/mnth.

My question is.. how do they know if you have tethered or not? Will they really know?