Identify invoice header texts in runtime before document saved

Hello ABAP GURU's    

User enters header texts in invoice using transaction VF02. We need to find those texts in runtime of program before invoice saves.Because we need to fill VBRK field with serial number when texts are entered.

When user re-enter or changes those header texts in the same object 2nd time, then we need to compare this texts with first time entered text to know whether text changed or new text entered. When we find texts are entered or changed 2nd time, then we need to fill VBRK table field with next number from series of serial numbers.

Here the question is, where do we find entered header texts in runtime of program before document saved? which part of program code/BADI/exit/BAPI carriers these texts in runtime?

What is the variable/internal table/workarea/field which carries these header texts in runtime?

Thanks in advance.