I have added a compare schematic feature in xschem. With an open schematic you give the name of a second schematic to compare with. The differences are shown. In grey the elements of schematic 1 (the one in the window) that are different or non-existent (in the indicated position) in schematic 2, in red the elements of schematic 2 that are non existent (in the indicated position) on schematic 1. The red shapes are just a ghost overlay to show the differences. On redraw these disappear. Once the name of the 2nd schematic is set, pressing Alt-x will compare again. You may zoom to a specific part of the schematic, press Alt-x to see the differences. The compare applies to nets and instances, any difference is flagged, including attribute differences.
For now the compare is appled only to nets and instances, the things that affect a netlist. May be in the next days i will extend the compare to everything , including non electrically significant shapes and texts.