Hint, Check patch history

One of the method to check DB patch history is query registry$history

SYS>select ACTION_TIME,ACTION,NAMESPACE,VERSION,BUNDLE_SERIES from registry$history

ACTION_TIME ACTION NAMESPACE VERSION BUNDLE_SERIES
------------------------------ ---------- ---------- ------------------------------ --------------------
12-SEP-08 10.28.24.895804 PM CPU SERVER 10.2.0.3.0
12-SEP-08 10.30.59.512031 PM CPU
30-MAY-09 11.34.14.809290 AM UPGRADE SERVER 10.2.0.4.0
30-MAY-09 01.13.24.879514 PM APPLY SERVER 10.2.0.4 CPU
30-MAY-09 01.21.04.024997 PM CPU