Gauge or sending unit?

[redsonfly]

My 18 has a full tank of fuel, the gauge has power and ground, but it reads empty; the needle does not budge.
How can one determine if it's the gauge or the sending unit that is the culprit? The problem is that this is a later tracker boat and they did not spec the tank correctly and you cannot access the sending unit without removing the tank panel in the cockpit sole.
Thanks in advance!