Can I find out when and towards what purchase it was applied?
You can view your account's purchase history, that will tell you how each item was paid for (e.g. 'Store Credit' if it used your account's balance). You can't view when gift cards were redeemed onto an account, though you could try contacting iTunes Support and see if they can/will tell you : iTunes Store - Contact Us - Apple Support (obviously they will only potentially be able to tell you if it was redeemed onto your account, they can't give details if it was redeemed onto somebody else's)