Can the phone network tell whether the phone or the computer is the end user of the data?
Don't know, but they can restrict the capability of the device. With my original O2 sim in my iPhone I can use it as a hotspot but with my 3 sim (which does not include tethering in the contract) in that option is not available in the settings - so the sim can restrict the options available. Other 3 sim/tariff deals are available which do allow it.