My AC my furnace and my furnace fan will not come on.
I tested the thermostat and put wire r+w togeather then r to g then r to y turning the power on eachg time.
Nothing works so I put the thermostat back in place. The fan on the furnace works when I put it on manually. How can I tell if there is a problem with the wire from the thermostat to the fan etc?

You will have to test the circuits with an electric meter. If you have a meter and know how to use it you are all set. If not call for local service.