Use the Show IP entry in the RetroPie system and it will show you the available network adapters, with their MAC:
...
3: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DORMANT group default qlen 1000
link/ether dc:a6:mm:aa:cc brd ff:ff:ff:ff:ff:ff