Finding whether RAC is enabled or not

To check whether the instance is RAC enabled or not
SQL> sho parameter CLUSTER_DATABASE;

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
cluster_database                     boolean     TRUE
cluster_database_instances           integer     4
SQL>
SQL> select * from v$active_instances;

INST_NUMBER
-----------
INST_NAME
--------------------------------------------------------------------------------
          1
Stage1

          2
Stage2

          3
Stage3


INST_NUMBER
-----------
INST_NAME
--------------------------------------------------------------------------------
          4
Stage4

Note: Here if the parameter value shows "TRUE" means that the node is RAC.
and if the above select command shows rows ..that means RAC is enabled to that instance.

Or else you can check in init.ora file from $ORACLE_HOME/dbs

*.cluster_database=false

Note: If the cluster database set to "false" --RAC is disabled.