Hi Mohan,
For user-exits:
1. search by string "call customer-function ". you will find an FM like call customer-function '008'.
inside this there would be a z-include. Put break point on each FM and check if that is the place where you have to write your code.
2. T.Code : SMOD.
Find the package of your program and search for user-exits in SMOD transaction.
Enhancements:
1.In your report EDIT-->ENHANCEMENT OPERATIONS-->SHOW IMPLICIT ENHANCEMENT OPTIONS.
2.T.Code : SMOD.
refer more:
Enhancement in SAP ( how to find , work and implement ??? ) « SAP Ignite
Thanks,
Anil