Consuming a Previous Purchase

So, we're testing our app, and we're noticing that some users are failing to receive the callback for their consumable purchase. What I'd like to know is... what options are there for getting a list of purchased consumables that haven't been consumed yet?

We'd like to get that list when users start the app and then give them their purchased goods + properly Consume their purchase at that time. In addition, we're looking into ways to double-check it happened and do that after a purchase fails its callback.