There is only the one sensor: the fuel level sender in the tank. The "gallons used" in the DIC does not use the sender value, but the "miles remaining" uses the value to detect a fuel fill and fuel remaining. Check this value against what it should be after a full fill. If it seems about right with your average MPG (for previous 50 miles) and actual fuel remaining in the tank, the gage is probably the problem. If it calculates well short, then the gage is likely suspect, but that may not be accurate if it isn't obvious, such as missing 1/4 tank of fuel while parked or stable driving on a level surface.

If that doesn't help......
If it is indicating 1/4 tank less than (actual) full to empty, but is relatively linear, the gage or sender is suspect. You'll need to check the datastream fuel level % value with a Tech II or similar scanner. If it matches the gage indication, the sender is failed/failing. If it tracks the fuel level in the tank, but is about 1/4 different than the gage, then the gage is failed.