When a column is marked unused, it is considered for all effective purposes as dropped — (so much so , you could follow up that command with adding a new column to the table with the same column name) — as I mentioned, this action (of setting column unused, is irreversible– there is no command to mark it “used”)….the only action left to do, is a space management operation to reclaim the space …so from this perspective, knowing which columns existed before, and were marked unused is irrelevant …all one needs to know is that table x has some columns that were marked unused (essentially dropped – minus space reclaim) …and that these tables are the ones for which space can be reclaimed via the alter table …drop unused columns command.
If you need to see which columns where marked ununsed , you can try one of the following approaches …
1 — View the 12.1.3 data model comparison report from MOS Doc. 1290886.1
2 — Try something like this in a test env which I found poking around in the web — which created a view on dictionary objects which supposedly does the column lookup even for unused cols — but I have not tried it : http://stackoverflow.com/questions/2949226/how-can-you-tell-which-columns-are-unused-in-all-tab-cols