This was the question of one of my colleague DBAs yesterday. Since he was on 10.2 and had AWR as a cost option we worked on DBA_HIST_ACTIVE_SESS_HISTORY first. After a while I decided to have the opinions of the experts at oracle-l and there Deepak Sharma advised a query based on DBA_HIST_SQLSTAT and Niall Litchfield advised DBMS_XPLAN.
So there is a guy who ran his query and insists that it took too long since he couldn’t get the DOP he requested that morning and he thinks this is DBA’s fault since he configured the Resource Manager not appropriate. As a result my DBA colleague wants to show that he really got the DOP what he requested for that query and he needs an accurate method to prove this on 10.2.
For the details of the discussion, it is still ongoing here and if you have any comments please let me know :)
Extract(Read) > Transform(Test&Apply) > Load(Learn) Blog 
The setting of parallel_adaptive_multi_user could also affect the DOP used.
You should be able to see whether a parallel operation was downgraded via v$sysstat. More info here:
http://oracledoug.com/px7.html
How was my parallel query executed last night? – AWR! – http://dioncho.wordpress.com/2009/02/20/how-was-my-parallel-query-executed-last-night-awr/