How to determine which sectors a file occupies on Windows?

Tools can use the Defrag API, specifically FSCTL_GET_RETRIEVAL_POINTERS, to get a list of extents (sector ranges) for a given file. You can use it on the command line through fsutil which comes built into Windows:

fsutil file queryextents C:\MyFile.ext

Various other third-party defragmentation apps also frequently have a function to analyze just a single file. For example, SysInternals Contig:

contig -a C:\MyFile.ext

As another example, Piriform Defraggler automatically lists the most fragmented files; selecting one file will show its locations in a graphical view and vice versa.

The Linux equivalent is the FIEMAP ioctl which returns a list of file "extents" (and the older FIBMAP which returns a list of blocks). It can be used through tools such as filefrag or xfs_io.