OK, divide and conquer.
Get a multimeter with alligator clips. Put it right on the starter bolt that feeds the high current power to the starter, then the frame of the car. If you have a built in solenoid it should read 12v. If you have an external solenoid, it might read 0 or 12v.
Try to start the car and watch the meter. What it does when it doesn't start will help tell you where to look.
If it drops to a volt or so and the starter doesn't crank, it's PROBABLY a battery/cable issue before where you're measuring. Start moving back towards the battery. If you measure the voltage right on the battery post and it drops and still doesn't start, it's gotta be something with the battery. As you move back toward the battery, if you suddenly see it STAY at
12v or at least stay somewhat high where previously it was diving to 0 you know you just passed the bad point.
If you have a solenoid in the starter, put the meter on the 'control' post for the solenoid. If you see it change when you try to start, chances are the problem is internal. If it doesn't change, then it's a problem somewhere else in the car like an interlock or even the key switch.