can I find out which columns are being used or not in a table?

[jdorlon]

How about putting a trigger on the table? I know there is no trigger for SELECT, but then what are the odds that the app does a SELECT but never an insert, update or delete?

make your trigger do this

if :new.column_of_interest <> :old.column_of_interest then
insert a row into some table for monitoring;
end if;

[omegamark]

If you are going for the all the user tables in the database then you are in for Trouble.

But for a specfic table means you can try by
1.sql_trace on
2.sql_text ananlysis
3.Try using the New Monitoring option for tables.

Identify the columns that you want to eleiminate.create a view on all the columns that you need .Test your application .

Then drop all the columns .This is going to be a pain.