1. You need to first setup the environment variable for ORACLE_HOME in your Path. But if you don't know that then simply change the directory to the place where Oracle Database has been installed.
2. Use the following command to view the installed top level components:
opatch lsinventory -all
RESULTS
Oracle Interim Patch Installer version 11.2.0.3.3
Copyright (c) 2012, Oracle Corporation. All rights reserved.
Oracle Home : /app/database/product/11.2.0/dbhome_1
Central Inventory : /home/fusion/oraInventory
from : /app/database/product/11.2.0/dbhome_1/oraInst.loc
OPatch version : 11.2.0.3.3
OUI version : 11.2.0.3.0
Log file location : /app/database/product/11.2.0/dbhome_1/cfgtoollogs/opatch/opatch2014-04-04_18-39-06PM_1.log
Lsinventory Output file location : /app/database/product/11.2.0/dbhome_1/cfgtoollogs/opatch/lsinv/lsinventory2014-04-04_18-39-06PM.txt
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List of Oracle Homes:
Name Location
OH1373473345 /app/fusion
OraDb11g_home1 /app/database/product/11.2.0/dbhome_1
OH246247651 /app/provisioning/tools/idmlcm