Look for a file in /usr/local/bin/ called oraenv. If it exists, you can run this shell and pass it your SID value. The script will set ORACLE_HOME and other variables. If the file doesnt exist, then contact your DBA or app support person to understand how they have setup the database. If you saw a bunch of Oracle processes running, then you know Oracle was installed and is running.

Example
. /usr/local/bin/oraenv testDB