There is no specific variable that is set when the email is sent. If you need to add in this to your existing Universal Email implementation one way to do it would be to alter the UE code just after the call to the mail function in php, or the sending of the message in the other server languages. Just after the sending you can set a variable to act as a flag to let you know that the email was sent, or at least the mail call has been made.
This is a custom implementation though that is not a supported feature. If you are using php you can just search through the mail_PHP.php file for the mail call and add in your variable after this. If you do it here just keep in mind that this page will get rewritten if you make any edits to your email server behavior.
