With the starter not mounted, connect all wires (the big battery cable and the solenoid wire). Use a jumper cable to give the starter a ground. Turn on the key and press the starter, or leave the solenoid wire off and use a jumper from the battery cable to the solenoid spade. If the starter doesn't turn, and all other components are good, then it is a bad starter. Or bench test it using a car battery in a similar fashion.