Even with image tracking it will be tough with Yahoo since the images probably won't be rendered. The best trick I can come up with, if you have a web server of your own, is include a link to download an image that only that recipient will ever get and see if your access log shows activity from them after sending the email. It's not perfect, since they could still be blocking the email, or reading it but not allowing images or clicking links, but those are the best options to try.