to find a workitem linked to an object use function module sap_wapi_workitems_to_object
to read the result use function module sap_wapi_read_container and read the simple container returned table - element '_result' or '_wi_result' usually 0001 in approve and 0002 is reject if this is a decision task.
if the request was approved after the workflow ended then the approval status will not be in the workflow, look for the request data tables.