Check the length of the flower stalk.
This page gives a clue to the difference. It says V. persica "ID: Flowers solitary in leaf-axils, on stalks that are longer than their leaf but not twice as long. Flowers blue, the lowest petal often white. The similar V agrestis and V polita have flower-stalks shorter than their leaves and flowers less than 8 mm across. V filiformis has flower-stalks much longer than twice leaf length."
http://www.plant-identificatio...
