Oracle publishes extensive information on MetaLink regarding patches and patch prerequisites. The tools AutoPatch, AD Administration, and other AD utilities can make patching easier. However, each site needs to have standard procedures for patching, and keep track of the patches which are applied to each instance. (You DO have a development, a test and a production instance, don't you?)
For information about how to use these utilities, refer to the AD Utilities Reference Guide, Release 11i (11.5.7).
As an Oracle Applications user, you may want to check out the applications diagnostics.sql files. For each of the core applications, there are some routines which identify from the database what patch sets and individual patches have been applied.
This was first published in June 2004