RAC Master node:

How do I determine which node in the cluster is the "Master" node?
For the cluster synchronization service (CSS), the master can be found by searching ORACLE_HOME/log/nodename/cssd/ocssd.log where it is either the Oracle HOME for the Oracle Clusterware (this is the Grid Infrastructure home in Oracle Database 11g Release 2).

For master of a enqueue resource with Oracle RAC, you can select from v$ges_resource. There should be a master_node column.