Get a one liter water or pop bottle. No tape over any holes. Fill it with water. Pour it in. See when water comes out. Keep taping and adding water until you know tail tank capacity.

As Mike said, SH dump valves have issues with the seal as time marches on. Rather than cutting holes in the wings mid season, put each wing on a stand, root end low, put 5 or so gallons of water in the tank. Check actuator on root rib for leak. If one leaks, you can take the drive loose, and pack grease (or toilet bowl wax) in around the shaft and put off cutting holes and replacing parts for a while. Take a picture of the fitting, spring, etc, before you take it apart. Not that hard to do.

Consider adding a drain hole to the fuselage. Should probably be at close to 3/8th inch diameter, as anything smaller can and will plug up with dirt and other stuff that just naturally gets into the fuselage. This will help keep you from taking a bath in your plane should another leak happen.

Steve Leonard