Oracle: Am I on a RAC? What query can I perform to determine if I am running on a RAC system?  If I'm on a RAC, I need to query from GV$* rather than V$*.
You can check the value of the CLUSTER_DATABASE parameter:

SQL&gt; select name, value from v$parameter where name = 'cluster_database';

NAME                  VALUE
--------------------- ---------------------
cluster_database      TRUE 


According to the Oracle Database Reference:


  CLUSTER_DATABASE is an Oracle RAC parameter that specifies whether or not Oracle RAC is enabled.