You can use a LAN / subnet scanner, like "Advanced IP Scanner" for Windows.
Or Fing for Android or iOS:
If you don't find the IP or MAC of a device in the Google Home list, it's probably not connected anymore, and should be removed automatically after some time. But this doesn't always work, and you can't manually remove devices from the list yourself.
You can however get a "fresh start" by factoey reseting all Nest Wifi devices and set it up from scratch. Use the same network name (SSID) and password, then your devices will reconnect to the new network automagically.