how to tell if fan clutch needs replaced

The fan clutch on a truck driven of the engine either fails and causes the engine to run hot (the gel in the hub leaks out and no longer is able to expand and "lock up" to cool the engine the other way it fails is it "lock up" and fails to release causing a load noise all the time. Its function does not affect a/c output.Poor a/c operation is normally low refrigerant charge.