Afaik there is no function to get the active pledges, ne would need to iterate over all quests and then somehow mark them if possible.
Ther eis an addon which is able to abandon the pledge quests for you and it shows a dialog asking you so you can easily check if you got a pledges quest with it and abandon it to get new quests too:
http://www.esoui.com/downloads/info1...dgeQuests.html