Hi Suresh,
You have an importing parameter in PROCESS_ITEM Method ,you can get the req. data using the methods of that object class .
DATA: lsitem TYPE mepoitem.
lsitem = im_item->get_data( ).
Now use the lsitem and write you validations.similarly in Check you have IM_HEADER object ,
Example for using the process_header
data : ls_header type MEPOHEADER.
data : lt_items_ref type PURCHASE_ORDER_ITEMS.
ls_header = im_header->get_data( ).
lt_items_ref = im_header->get_items( ).
Check and revert.
Regards,
Pavan