1. Download the Amazon Alexa app to your phone or computer and sign in with your Amazon account
2. From the Alexa app, click on gear icon at bottom
3. Select your device
4. Scroll to bottom of the page and view MAC Address.

Amazon Alexa support site

1. If your device has been used on a different Wi-Fi network in the past, perform a Factory Reset before continuing (press and hold microphone mute for 15 sec)
2. Open the Google Home app
3. Follow setup instructions for your device
4. When you reach the Connect to WiFi step, tap the three dots in the upper right corner and choose View MAC Address.

Google Home support site