How to confirm that the latest PSU has been applied to your ORACLE_HOME

Use opatch lsinventory to extract the information. In my case:

hostname:PRODDB01>opatch lsinventory | egrep -i 'PSU|DATABASE PATCH SET UPDATE'

Patch description:  "Database Patch Set Update : 11.2.0.4.2 (18031668)" 
Sub-patch  17478514; "Database Patch Set Update : 11.2.0.4.1 (17478514)"
Patch description:  "Database Patch Set Update : 11.2.0.3.7 (16619892)"
Sub-patch  16056266; "Database Patch Set Update : 11.2.0.3.6 (16056266)"
Sub-patch  14727310; "Database Patch Set Update : 11.2.0.3.5 (14727310)"
Sub-patch  14275605; "Database Patch Set Update : 11.2.0.3.4 (14275605)"
Sub-patch  13923374; "Database Patch Set Update : 11.2.0.3.3 (13923374)"
Sub-patch  13696216; "Database Patch Set Update : 11.2.0.3.2 (13696216)"
Sub-patch  13343438; "Database Patch Set Update : 11.2.0.3.1 (13343438)"