Quick and easy way to check this out is to look on the back of your fuselage.

Sorry but the best photo that I have of a rear part of the fuselage had to have a cute blonde in it as well!
If you look even with the rear of the wing, you will see the fuel cap for the aux tank. 90% of the time its on the right side but I have seen them on the left hand side on some airplanes. If you have the cap on the outside, then you have an aux tank installed.
Joe A