How do you test a solenoid on a riding mower?

The solenoid will have two big posts and one or two small posts. Mark and remove the wires from the small posts. If you have one small post you can hook up a jumper wire from the + side of the battery to the small post and it should make the starter crank over. If it has two small posts hook one jumper from the + side of the battery to one of the small posts and another jumper from the - side of the battery to the other small post. If it is OK it will engage the starter. If you get nothing then the solenoid is bad.