My Nighthawk has a gas gauge, but it would not adapt well to the Shadow. But it does work reasonable well. Honda should have put a voltage regulator on it though, since the display of the gas in the tank goes up and down depending on how much of a charge the battery has!
But, that aside, it shouldn't be too hard to put a T fitting on the hose coming out of the petcock (provided you don't have a vacuum-actuated one!) and a T on the vent hose going to the tank, and connecting the two of them with some gasoline-safe clear tubing. Run the tubing out toward the triple-tree, and make sure that it runs vertically froma point below the tank to a point above it.
When fuel is in the tank, the level in the clear tubing will show you how high the fuel level is in the tank. From there, you can figure out about how much you have.
A word of caution though: a motorcycle fuel tank is an irregular shape. It doesn't have a (roughly) volumetric constant like a car tank does. "Half full" on a motorcycle fuel gauge is not a linear progression toward the number of miles that you can go. Some parts of the tank will drop vertically much, much faster than other parts even if fuel consumption is constant. So, for that reason, a fuel gauge on a motorcycle is not going to be as reliable a method for determining your riding range as using the trip meter, as others have suggested. Also, remember that the bottom 1/3 of a motorcycle fuel tank is typically the reserve, and so we need to be sure not to count that is viable fuel when making a fuel gauge of any kind.
--Justin