Just use a test light or meter. With the key in the start position, you should have voltage to the small trigger wire, as well as battery voltage to both of the large posts. If no voltage to the small wire, then your problem is before the solenoid. If you have voltage to all 3 with the key turned, then you have either a bad connection on the starter or ground.
There is also a solenoid on the starter itself. so it could be either one.