Most mobile network operators state that tethering is against their ToS (which I don't like; if I pay for 1GB data allowance should I not be able to use this where I want if I stay within my limit?)

So I'm curious as to just how they detect tethering?
Is it only if you take your sim card out and place it into a dongle, or can they detect if I'm using my phone as a modem to access the internet over wifi?


Note: I'm not advocating anything illegal, just curious.