To check serial number field emptiness.

Hello experts,

               I have a requirement where, in  MIGO if Serial number doesn't exist in the field of serial number, new  customized serial number must  auto-generated and populated into field of serial number.  I am using badi method " IF_EX_MB_DOCUMENT_BADI~MB_DOCUMENT_UPDATE."

In which table can i check if  serial number is empty? Is there a way to extract this serial number using material , quantity , plant .

Thanks in advance,

Prakrita Kapoor.