My Huskee 20 HP, 50 inch cut, riding mower won't start. I tried 3 different batteries, the last one is brand new. All with the same result. I just hear a click when the key is turned on. A friend said that it could be the solenoid. How do I test it? Thank you in advance.

Hi,
If you hear it click and it does nothing, it quite likely is the solenoid and the high current contacts within it are damaged and not forming a connection. Placing a screwdriver blade across the large terminals will serve to act as those contacts and the engine should spin over. That would verify that situation.
Peace,
Clarke