Adding to Michael's comment which is very helpful in 11i/r12, you can further analyze a patch by having clear understanding of below files -
1. Driver file - u<patch#>.drv where in you should look out for -
a. copy portion
b. apply portion
c. generate portion
Moreover, there is one more file - f<patch#>.ldt which will give you list of files that will be modified by a particular patch.
Thanks a lot