Here's a simple test. Take the blue power wire connecting to the sending unit, and ground it. In the picture, I took a clip-lead and connected the wire to the ground strap on the sending unit pick-up tube. In this test, your gauge should read full when you turn on the key. If so, then the sending unit is bad. If it still doesn't read full, then either power isn't getting back there, or the ground strap isn't making a good ground anymore.
Mike