The way I research DPO's is to go to the USPS site tab "find a post office." If enter a town and state and the nearest post office is not in that town, it would be safe to assume that post office has been discontinued.
Also, for me, DPO does not so much mean Discontinued Post Office as Dead Post Office. That's the first thing that comes to mind when I see DPO. After all, who thinks DOA means Discontinued On Arrival?
