Well, do they allow tethering? If not, they could potentially tell by looking at your past data usage. For example, if you usually only use 100MB a month, but start tethering and go up to 1-2GB per month, they may call you on it.
Read the contract and if it says nothing about data caps or tethering then whats the worst they can do...Stop letting you give them your money? If they do have a tethering they probably get mad but if not its a little easier.