What's this last device in AirPort Utility's Wireless Clients list?
To answer your question, I don't think that there is a definitive way to tell what is on your Wi-Fi connection, at least not without additional software (if such software exists). That said, here's what I would do to rule out products that I or my family own:

The MAC address pictured looked similar to my Apple Watch, with an Apple Watch very easily overlooked on a network. Like I said in the comments, it could be an Apple Watch, to test it just put the Apple Watch in Airplane Mode 



and see if that mysterious MAC address disappears. Even if that's not it, you can test every device in your house this way.