what about streaming in the file into a dummy-library and checking the log- and summary-files?
strmin -library MYDUMMLIB -strmFile filetocheck.gds2 -summaryFile ./gds2streamsummary01.txt
If your library defines all possible GDS2-layers and -datatypes, you should find all informations
in the summary-file;
If no layer at all is defined in your library, you should get all informations as warnings in the strmIn.log,
and for everything inbetween, you'd have to check both files to get the complete information.
Besides of Cadence, you could create a small Perl-Script using the GDS2-module or you
could perhaps use a scriptable GDS2-viewer to do the job.