If you jump the solenoid you need to be careful since it can spin engine over and tractor could run over you and injure you.
But there should be a small wire (black with white stripe?) attached to a spade terminal on the solenoid. Remove that wire then attach a test light or volt meter to that wire then turn the key to the start position with all safety switches in the correct position to allow starting (same as you have been successfully starting it). If there is voltage to that then that shows everything is working properly and the starter should be working correctly.
Then to verify that the starter and solenoid are operating correctly you will need to apply voltage directly to the spade terminal. Use a wire directly from the positive battery terminal to the terminal. Use a 12 gauge wire or something similar, just not too small. Tractor engine will crank if the starter and solenoid are operating properly.
Between these two tests you should know if it is wiring problems or starter problem.
There is a common problem with the wiring and safety switches where there might be voltage at the solenoid terminal but there has been enough voltage drop that the solenoid won’t work. That problem is solved by installing a relay that is operated by the solenoid wire but closes and supplies voltage directly from the battery. See this schematic.
