Fuel gauge senders (and most oil pressure and water temperature senders) supply a ground to the gauge. You should not be able to measure power at the sender when it is connected.
To check the sender, you measure resistance to ground with a meter. 0 ohms should be either 100% full or 100% empty with the harness disconnected. Usually 60 or 90 ohms is full or empty. A half full tank should measure around 30 to 50 ohms.
If you ground the sender wire, the gauge should swing to either full or empty depending on how the system was designed.
These checks should narrow the problem to either the sender or gauge. If both check out, it may be a broken wire in the harness between the two.