how to check whether Oracle database is accessed or not?
Take a look at it this thread on AskTom:


  Auditing of course would be the easiest way
  
  You could possibly get something from the DBA_HIST views (part of AWR/ASH - workload repository and active session history) but bear in mind
  
  
  they are sampled snapshots - not 100% of everything is in there
  many of them are "top" reports only - dba_hist_seg_stat for example - shows IO's against segments - it seems perfect, but it is only "This view captures the top segments based on a set of criteria and captures information from V$SEGSTAT" - it will not have 100% of everything.
  
  
  Auditing is the right answer, funny thing with auditing - we only really want it when it isn't on don't we :)


Oracle has a lot of history tables as you can see here:
http://www.dba-oracle.com/oracle10g_tuning/t_dba_hist_data_dictionary_views.htm