2005 express fuel gauge shows emrty all the time? how do i know if its the sending unit or just a bad gauge

You have to back track the wire from the sending unit to the guage. If there is no power when you put your volt meter to the wire and the body you should get movement. There could be a break in the wire, or it could be a bad guage or a sending unit. obviously the sending unit is the hardest to fix. So start with the wire, then the guage then the sending unit. If you do replace the sending unit. I personaly would replace the wire no matter what. One solid wire with a good heat and weather proof coating.