Use a multi-meter. If you have full battery voltage to the fan wires, check to see if the fan blades can turn at all, it might be seized for one reason or another. Set the meter to ohms and read the resistance (remove the fuse for the fan), if it doesn't read anything then it's open (read bad). When you touch the leads of the meter together it will read 0 as in no resistance. When you have the leads in the air the meter will usually have a 1 in the left side indicating that there isn't a connection between the 2 leads.
Jeff