$_product = ...; // some product object you already have

$_productCollection = Mage::helper('wishlist')->getProductCollection()
    ->addFieldToFilter('sku', $_product->getSku());

if($_productCollection->count() > 0) {
    // User already has item in wishlist.
}

public function isInWishlist($item)
{

    $_productCollection = Mage::helper('wishlist')->getProductCollection()
        ->addFieldToFilter('sku', $item->getSku());

    if($_productCollection->count()) {
        return true;
    }

    return false;
}

if ($this->helper('wishlist')->isInWishlist($_product)) :

function checkInWishilist($_product){


Mage::getSingleton('customer/session')->isLoggedIn();
$session = Mage::getSingleton('customer/session');
$cidData = $session->isLoggedIn();
$customer_id = $session->getId();

if($customer_id){
 $wishlist = Mage::getModel('wishlist/item')->getCollection();
  $wishlist->getSelect()
            ->join(array('t2' => 'wishlist'),
                   'main_table.wishlist_id = t2.wishlist_id',
                   array('wishlist_id','customer_id'))
                   ->where('main_table.product_id = '.$_product->getId().' AND t2.customer_id='.$customer_id);

             $count = $wishlist->count();

 $wishlist = Mage::getModel('wishlist/item')->getCollection();
}
else{
    $count="0";
}

    if($count) :
    return true;
    else:
    return false;
    endif;

}

<?php $wishlist = Mage::getModel('wishlist/item')->load($_product->getId(),'product_id');
      if($wishlist->getId())
          //product is added
      echo "Added! - Product is in the wishlist!";
      else
          //add product to wishlist
      echo "<a href='".$this->helper('wishlist')->getAddUrl($_product) ."'>Add This?</a>";
  ;?>