How do I find out the address of [Settings>Display->Cast] devices? I see a [TV] Samsung 6 Series (50) wireless display in the list from display settings. But I do not own a Samsung television, so I would like to know the IP or MAC address of this device to double check with my router's list of connected devices.
You can't easily identify its MAC address as its not connected to any AP.

Wireless display (WiDi) does not require any Wi-Fi Access Point or router or hotspot for operation. It uses WiFi Direct® (or Wi-Fi CERTIFIED™ Miracast®) technology, for wireless connection to WiDi compliant receiver.

Having said that, it therefore implies that by your device identifying [TV] Samsung 6 Series (50) in Settings→ Display →Cast is not connected to any AP, but it just being detected by your device as a "nearby device" and does not even reveal its MAC/IP explicitly even if you make the connection.



However the easiest way to identify its MAC/IP is allowing it to connect to any of available wireless AP:


Go to your TV's menu and choose Network → Network Settings
Choose Wireless for your connection type
Choose to connect to the  SSID
At this point, the Smart TVs will show you a screen with the Wireless MAC address.
If not, go back to the Menu and choose Network → Network Status. 


Alternatively check on your device's list of connected devices and get the IP.



Note: I have even observed that my Smart TV (Samsung) shows in the list of Available Wireless Display devices on my tablet, even before connection.