Hi Harshal,
Basically the WBS element will exists in EKBN table in purchase requisitions. You can validate the Right WBS element by using the below BADI. check the methods which are relevant for MRP planning of PREQ's. you can have the import and changing parameters of EBKN and EBAN
IM_EBAN_OLD
IM_EBKN_OLD
IM_VSTAT
CH_EBAN
CH_EBKN
MD_PURREQ_CHANGE
CHANGE_BEFORE_SAVE_MRP
CHANGE_BEFORE_SAVE_CONV