There is no way for them to tell, they would have to do individual packet inspections, time consuming and costly. AT&T can't actually tell. They would have to inspect port 80/443 traffic look for the browser used i.e a mobile browser, other then that there is nothing to distinguish between a laptop or mobile. They just use scare tactics to get you to pay the extra charges. If you are very paranoid you can just setup a vpn tunnel, no possible way at all for them to inspect your traffic. If you started to use an outrageous amount of data, I would assume you would get flagged and someone would contact you. But Three does not have anything in the T&C's about tethering.
So all good, plus their network is not good so it is hard to rack up a lot of data, I use about 5G per month on mine just streaming radios stations, I dock the iphone and run it through my receiver at home. Just to get the value out of my unlimited pack :-)