This is the simple tricks but useful for you. because sometime you need to work in an user exits in SAP Standard program, for example you want to know all of user exits in ME23N SAP Transaction code. you can do on following way.
Go to ME23N Transaction code.
After ME23N SAP Program opened, you can go to Menu System->Status, to get Program name.
ME23N SAP Transaction code have the program name is SAPLMEGUI, Now you can go to SE38 Transaction code to get Package name of program. On SE38 SAP Transaction code navigate to Menu Goto->Attributes.
The Program SAPLMEGUI have the Package – ME. After you know the package you can go to SMOD SAP Transaction code. Go to menu Utilities->Find and enter ME in package field.
And click Execute button on toolbar. After that you will get All of User Exit in ME Package, you can choose any appropriate user exit for your requirement or case.