Tobbera's suggestion gave me a great boost to solve my problem.
Material:
- One of those cheap 240Vac "leds" (see image)
http://tinyurl.com/mbys9ah
-A simple fotoresistor (i had a few around) - A 10K resistor
The led terminals are connected to the coffee machine breaker switch (240Vac).
The led and the fototransistor are optically connected (and secured using simple black isolator tape).
The 10k resistor forms a voltage divider with the fotoresistor, which is connected to the analog in of the Arduino
Secure and cheap solution.
Thank you for all the replies.