Detect if image has been used as featured image

After using get_posts to get some attachments I can check post_parent to find out if they have been used in a post. However this does not work for images I used as featured image where post_parent returns 0 (0 means that the image has not been used).

Is there a way to detect if an image has been used not only in a post, but also as the featured image.

Thank you very much in advance!