SKPaymentTransaction: How to get paymentQueue.updatedTransactions called when needed (or access the user transactions)?

Question

I would like to have the ability to check whenever I want that the user made an action related to his transactions, i.e. getting a refund outside of the app.

I am saving the SKTransaction's transactionIdentifier in the UserDefaults when he completes a transaction, however once this is done I have no idea how I can trigger a call to paymentQueue(queue, updatedTransactions) (or whatever function that could let me know the user's transactions state).

Thank you for your help