You can navigate into your game folder, find the SH4.exe file, right-click on it, then left-click on "Properties", look on the Details tab. About the middle of the page will be "Product version: 1, x, x, x", with the "x" there representing numbers. 1, 5 is version 1.5 with the U-Boats. That is also another way to tell which version you have, is if you have the U-Boats Career option, then you have v1.5...