How to tell if your database is RAC? (Oracle)

You are a consultant and just got paged to a database issue from a new client. Their DBA is on vacation and this is your first time to logon to their system. How can you tell if this is a RAC database? If it is RAC, how many instances/nodes? Where are the other nodes?

• check if it is a RAC database

          show parameter cluster
          --if it is RAC, cluster_database should be "true".
          --cluster_database_instances would show you how many nodes of it is a RAC

• find IPs of other nodes

          cat /etc/hosts
          --this command should tell you IPs of other nodes