Starter motor test.
If you are at all unsure about "messing" with electrics, then give this a miss.
Find the two "thick" wires that go to the starter solenoid, and "bridge" them with something suitable. I use a pound coin when I "bridge" the one on the trike. A jump lead can also be used. Do not use "normal" thin wire, as it will probably get red hot and burn a nice little hole in your fingers.
When you bridge the solenoid, the starter motor should spin, if it doesn't, then you will have to remove the starter motor and check it. Possible causes of failure are brushes worn, windings burnt out or the shaft has seized.