There is no method if the video has been send as attachment. All you can do is using some kind of
Email tracking - Wikipedia[
^].
If the mail contains a link to the video and it is stored on a server controlled by you, you can provide a personalised link and check the server logs for an access to that link.
Finally, even if you got some kind of feedback it will at best indicate that playing the video might have been started. But you will never know if it has been viewed.