title: Check whether a product is in the wishlist or not
body: <p>I'm working on a Magento theme, and I need to build a function that can check to see whether or not a product has been added to the user's wishlist. </p>
  
  <p>Magento has a "Mage_Wishlist_Helper_Data" helper class, but I have no idea how to build a check-if-already-in-wishlist function. Basically I need to use Magento's wishlist feature to build a favorites list. I want to add a special class to the "add to wishlist" link if the particular product was already added to the user's favorites.</p>