Try opening up Lr and checking.
I recall another user having a problem, and it seemed that he got a truncated catalog, as if x percent was copied and imported, so he was able to resume. Which would be the case if the importer acts like the regular importer, and is copying, adding the info to Lr, then moving on. In any case you could start over after deleting the Lr catalog and copied files that Lr's importer created; it wouldn't have deleted anything from your existing photos. Might be faster than trying to compare to see if thousands of photos are all there.