Paul__B:
Opto isolator with the current limited by a capacitor - likely 0.22uF rated 330VAC with a series current-limiting/ protective resistor of about 2k7 - and either a bridge rectifier, or a reverse diode across the isolator LED or an AC isolator with back-to-back LEDs internally.
I would also go with optocupler. Gives you total isolation between the high volatges in the coffemaker and your sensitive Arduino.
Another way would be to build your own optocoupler. Is there a light that turns on when the coffemaker is on? Then place a light sensor on ,or close to, that light and connect it to your arduino. Viola! =)