How could you check a gas valve to see if is working OK??

You need to put all your posts under the same original one. I cannot follow the thread. Depends. When it starts up you measure the voltage going to the valve. Usually 24 volts. If there is 24 volts at the valve then the gas valve is not opening and is defective. Since you really do not know about furnaces you might be better served by calling in a HVAC professional and have them check it out.