Usually if you remove the hot wire least the sending unit and touch it to ground the gauge should read full. If it does that tells you the gauge is OK and the trouble is the sending unit.

You don't say what year Ford you have, but assuming the dual tanks and sending units are factory installed I will guess it is a late 60's or newer Ford pick-up. There are numerous sites that cater to Ford trucks. Any of these sites will be able to delve deeper into the problem you describe. A constant "full" reading could indicate a problem with any part of the fuel gauge circuitry, though a sending unit problem usually reads "empty" rather than "full". Here is a quick check to check out your sending unit. Check the sending unit by remove the wire from the sending unit and then check the gauge. Make sure to remove the "hot" wire, not the ground wire, if equipped. If the sending unit is at fault the gauge will drop to empty, returning to full when the wire is reconnected. If the gauge stays at full with the wire disconnected from the sending unit that indicates a bad gauge or a short in the wire from the sending unit to the gauge.