Open your assembly and part files, make sure the coordinate system of each is at absolute. Go to the part file and save a copy of the part's absolute coordinate system (WCS->Save) then export that to the assembly file (or add it to a temporary reference set in the part file). Now the difference in the assembly's absolute coordinate system and the part's coordinate system is the amount that it has been repositioned.
There may be a cleaner method to go about this, but it is the first thing that comes to mind.