you can with a test light again, touch the little stud with the purple wire on the starter only, then have that helper turn the key to crank, when the key is turned to crank that little wire should have 12 volts to it. if it dont have power it could be ignition harness related.

if you bridge the little stud and the pos cable's stud with a screwdriver, thats called jumping the starter, it will crank if the starter works, regardless if the key is off or on.