Jim, all they really have to do is look at the device id on usb or the bluetooth address. Because they know which models use what id and what hardware was used for the devices it wouldn't be difficult at all. Not saying thats what they did but I don't think it can be discounted that a driver installed on the client machine would be able to know when a device isn't supported for tethering. For that matter the same method could be implored in the misd deamon without trying to go down the sticky road of detecting a 3G connection.

