When you leave a group on any of the applications of this kind, all users will be notified in different ways. For example, in WhatsApp, a message is posted in a different way where all users can see, but you have to actually enter the conversation. I believe Viber notifies it in the same way, which means that you should look in the group's conversation between messages. It should say Number/Name has left the group.
This is the only way to check if someone actually left the group conversation.