Check permanent live feed (thick wire) into starter, should be 12v

Check switch feed (thin wire) into starter, should be 12v when cranked.

Check the positive wire on the solenoid, should be 12 volts with ignition off, zero volts when cranking. If you get any voltage over 0.3v check for voltage drop around the circuit. If that is the only place volt drop occurs suspect worn out solenoid.

If any 12v feed is not there suspect broken circuit.

Check the earth between the starter body and battery, any voltage reading when cranking suspect earth problem. Watch out for the starter attempting to earth through other wires, especially the throttle cable. Check the throttle cable to see if it is hot.

Volt Drop Tests

Volt droping is simply checking to see that the voltage is going were it should. For example there should never be any voltage on an earth wire when the component is being used as all the voltage should be used up by the component.