Video Embed Field: Check All My Videos on existance? My question on drupal.org did not have an answer:
To check if YouTube video exists in all my nodes?
Please yow to do these?
the quickest solution is to query the DB and export all youtube video IDs. 
After you can write a custom PHP script to check if the video exists.
Here is an example of the code you could use: https://stackoverflow.com/a/21069917

A more Drupalistic approach is to write a custom module which do the same thing. For example it could implement a drupal cron task or a batch procedure to verify each Youtube field content using the code suggested before.

I hope this suggestion will be helpful.