The WiFi module on Marty's Rick control board is an Espressif ESP8266. This module has two MAC addresses, one that is used when it behaves as a Client connected to another network, and the other which is used in the Setup Hotspot, or Access Point mode.
The Access Point MAC address is the same as the Client MAC, except the highest octet is +2. For example:
Where in hexadecimal
To Find the Hotspot MAC Address
Generally, the Access Point MAC is easier to find if you're setting up a new Marty. During setup, when your phone or tablet is connected to the Marty Setup network, you can point a web browser at http://192.168.4.1/i which will display a bunch of information about the board.
You can also use an IP/ARP scanning tool such as Fing to get the MAC address of the Hotspot
The MAC will be shown under the router/hub icon:
If you're needing the Client MAC, remember to subtract two from the leading octet!
To find the Client MAC Address
Marty needs to be successfully connected to a network or the client MAC to be discover-able, hence why the above Hotspot method is sometimes useful. You can again use Fing or many other IP scanning or ARP scanning tools to find the MAC addresses of devices, including Marty, attached to the network.