Sold Out image on product listing

So I've been asked to put in a sold out sticker over the product image when it's.....well sold out. Now doing this in css would be easy enough to layer the new image. However I cannot for my life figure out how to check if a product is at 0 quantity or not on my product listing pages.

I looked in to the tpl_modules_main_product_image file and was able to check there and display a custom picture however that is not called on the listing pages only on the product_info page where a sold out tag is already displayed. From what I can gather the information on the listing pages is from the $list_box_content variable. But I'm unsure how to check this for quantity on each product.

Any insite would be appreciated. Many thanks in advance.

In the product_listing.php module, you can test the quantity with:
$listing->fields['products_quantity']

If it is <= 0, then it is sold out ...