You can a call to GetSellerEvents at a regular interval and it will return items that have a best offer. If the count changed since the last time you made the call, it implies that you have a new best offer. You get the details about the best offer for the item, you need to make a call to GetBestOffers with the ItemID.
Additionally, you can subscribe to the BestOffer notification event and get alerted when a best offer is made for your item.
Platform Notifications http://developer.ebay.com/Devzone/guides/ebayfeatures/Notifications/Notifications.html