DI API / SQL : How to find whether PO hyas been received and when?

Does anyone knows "the correct way" of finding out whether Purchase Order has been recieved and when? I tried of implement this using SQL queries run via DI API, but my solution does look smart to me as it retrieves information from number of tables and involves some logic.

Please share your experience. Thaks

P.S. If I find proper way of implementing this, I will post it into this tread.