Checking the voltage at the starter while the it is engaged is a good idea. that will tell you pretty quick what is going on if you can get to the connections. You want to hold the black lead to the starter case (bare metal) and the red (positive) lead to the terminal with the big battery cable. have partner turn the key and see how much you get. Should be at least 10.5 volts and more is better. If you have good voltage there, check the other big stud on the solenoid. This is the one with the short lead going into the starter. Want to see good voltage there when you turn the key to start. If good to solenoid but bad out, well ya got a bad solenoid! If you have 12+ volts at the starter with key off, but it drops way down when key turned to start, something up the line is bad. Check what the battery is doing in start position. If voltage drops off badly at battery in start, then battery is probably weak/bad. Another thought: Put the big socket on the crank bolt and try to slowly rotate the engine by hand. Should be hard, but not impossible. I had an alternator sieze up so bad the engine would not turn over on a diesel once. Just take belts loose if you come to suspect that.