My motor has an earth wire attached internally which feeds through the wire to the switch box, then to the rear power supply casing where it is earthed.
I'd expect yours to be similar.
If in doubt, stick a voltmeter between the motor case and mill. If you get 0.0v then it's earthed. If you don't, then worry.
The stock motor mounting on mine is also isolated. It's plastic with no metal-metal connection through.