How to find the extension structure of a given BAPI
For example lets take an example of BAPI_PO_CREATE1.
1) Find the package of the BAPI in which it is stored.
2) Goto SE11 and give BAPI_TE_* in the data type and press F4.
3) Click on structures
4) You will get a huge list of structures. click on new selection
5) In the pop up provide the package name from step 1 and press OK.
6) Now you will get a list of structures related to that BAPI in step1.