Follow your nose. Mine was a rust hole in the gas tank filler pipe that was concealed by the rubber surround. I think that a plugged gas filler drain hose allowed water to seep past the rubber around the filler neck and sit there. It wasn't until I filled the car up, drove around till the smell was bad, and jumped out to look for leaks that I found it. Looking up from underneath I saw a telltale wet spot on the filler neck where it passes through the rubber surround from the filler door. Sure enough, there was a pinhole rust-through up under the rubber. I copped a good used filler neck at the local Pick-a-part, dropped the tank, installed, and was good to go. I have also seen problems with the vapor recovery system result in a saturated, dripping charcoal canister. Also, you can activate the fuel pump by shorting the fuel pump lead on the test plug under the hood to ground (key on). That way you can pressurize the fuel lines at will without the motor running, so you can hunt down leaks there. I use an old-timey remote starter switch to do it, which allows me to carry the switch with me under the car while I hunt. Pretty handy.
