Fuel tank check
 

I currently am having a problem with a leak off the front of my 99 fuel tank. Is there any way to see if it's something other than the seal on the front? The tank leaks when there is over half a tank and seems to be OK when around a quarter tank.

My understanding is in the "rust belt" this is a common problem. Has anyone come up with a way of verifying it's the seam vs a line without dropping the tank? I ask as dropping the tank means dropping the entire rear sub frame.