If you are using SPM12b, there's an option in the Deformations Utility for generating Jacobian determinants from deformation fields.  There's no simple way I can think of for doing this in SPM8 though, although the operations involved are fairly simple.  Essentially, you want to warp one of the Jacobian maps, with scaling by the Jacobian of the deformation used for warping  (ie "modulation").