As A.B.Cada pointed out, you can query the database itself with sqlplus for the db version. That is the easiest way to findout what is the version of the db that is actively running. If there is more than one you will have to set the oracle_sid appropriately and run the query against each instance.
You can view /etc/oratab file to see what instance and what db home is used per instance. Its possible to have multiple version of oracle installed per server as well as multiple instances. The /etc/oratab file will list all instances and db home. From with the oracle db home you can run "opatch lsinventory" to find out what exaction version of the db is installed as well as any patches applied to that db installation.