User-exits will allow the users to add the additional functionality to the standard SAP programs. These standard SAP programs with user-exits contain subroutines and they have a pre-defined format to find . The prefix of the User-exit is USEREXIT. An user-exit contains the three character code which instructs the system to access a program.
If you want to find an user-exit for any standard transaction. For Example : 'VA01'.
Goto the transaction VA01.
Then Goto SYSTEM in the Menu bar and then to STATUS from where we can find the PROGRAMNAME.
Generally user-exits are always at the start of the standard SAP program.
The Other way to find the User-exits is by going to Transaction : SPRO
Go to transaction : SPRO
Here in these we can even find the documentation of the user-exit, for what purpose we can use.