Hello-
 dcm2niix with put the Siemens slice order information into the NIfTI slice_order tag. An exception to this is if your data was acquired with multiband, where the slice order can not be completely described by the 6 slice order options
 http://www.mccauslandcenter.sc.edu/crnl/tools/stc
In this case, you can look at the "SliceTiming" field of the BIDS json header to determine the order.