An easy way to check the solenoid. Check that you have 12V from the battery to the solenoid. Without activating the solenoid, check for 12V across the solenoid input and output terminals. This verifies your connection to the starter. With the solenoid engaged there should be a very low voltage across the terminals if the starter is drawing current. If you are still reading 12V with the solenoid activated, then the contacts are open or have a high resistance. You can also use your set of jumper cables to jump across the large solenoid terminals. The starter should run.
Carl