Your Service Now admins can check your system mailbox. If you have access to the record related to this email you might be able to find traces in the activity stream. In my instance, when a notification is sent from an INC it will show up in the activity stream saying "email sent". If you see that, then it's definitely a notification from SNOW. They would have had to script it so the reply to address is different then the From address. Otherwise, it would be from <your instance>@service-now.com. 

If you have access to the mailbox, did you check the sent items folder?