Splice a rheostat into the line and slowly turn it lower (higher
resistance) whilst driving.
-or-
See if you can find the wire harness leading to the tank... probably a
connector you can unhook.  The Fuel pump wont run either... but see if the
gauge still shows full.

Cheers.  

It's either a stuck sender float in the tank, or a short in the sender wire.
Pull the sender connection at the tank; if the gouge goes down, it's the sender
float. If the gauge stays up, there's a break in the sender wire, shorting to
body ground.

replace the fuel sending unit