Its is easy to test the fan clutch, if it gives quite a bit of resistance when motor is off when you try and spin it probably doing pretty good there but if its at highway speeds sounds like water pump/thermostat issue. The thermostat is either stuck open or shut with not enough pressure from water pump to circulate the water......if the temp gage is correct. Fan speed is off the temp cutout sensor on the front of the motor.

Like I was saying high temp with air passing radiator at high speed sounds like a thermostat and/or water pump issue.