They had a site where you could register your card, I think I had to do mine. Look on the packaging the card came with, or on the back of the card to see if it has more info on the website, but you can check it there for sure once you register the card.
Alternately, you can look at your past emails from zune and add up your purchases manually.
Note: that there is always room for user error. First, you could over count and not get the full $25 worth of your money. Second, you could under count and spend more then you have on the card. In that case, your account will be locked until you pay off the over balance. Which can be a pain (It let me buy a lot of apps on an old expired card that was linked to my acct from before I started getting my xbox live gold through prepaid cards. Then one day my acct was locked up until I put a valid card in and paid them off. To this day, Im not sure why they let it go through since it was expired for like 3 years)