If I understood correctly you need to find out the active / in Error workflow for PO APPROVAL.
Using the "Workflow administrator" responsibility navigate to Workflow monitor and you can put in criteria to view all active and in Error workflow.
Let me know if this helps or is it that you want to do it out side of the application.
You will have to query the wf_item_activity_statuses table and pick the ones which have errored out from a certain start date and end date.
SELECT * FROM WF_ITEM_ACTIVITY_STATUSES_V
WHERE ITEM_TYPE = 'REQAPPRV'
AND ITEM_KEY = :ITEMKEY
AND activity_display_name =<Use any other column if you wish>
AND ACTIVITY_TYPE_CODE = 'PROCESS'
AND activity_status_code = 'ERROR';