Check for voltage on a feed wire to the switch. If you find 12 volts on one of the wires, check the connector for signs of overheated and blackened terminals. If none of the wires have voltage, check the fusible link wires coming off the larger starter terminal by pulling on them. If a link is good it will act like a wire. If it's burned open it will act like a rubber band.
Another place to look if there's no voltage to the switch is for a corroded or overheated terminal in the bulkhead connector. In the case of a burned fuse link or overheated terminal, a digital voltmeter can be misleading. A test light will give more accurate results.