If you are looking for an automated report which XDPs use a specific fragment - that does not exist. A fragment is an individual XDP file which is referenced in n layout XDP files. You would have to write yourself some program which scans your layout XDPs and detect references to specific fragments.
if you are using the designer in standalone mode there is no way to tell where a particular fragment is used. If you are using interactive communications there is a way to find out how many documents use a particular xdp