Transforming 240 VAC to 5V/gnd
You can use an opto isolator and remember that isolating the AC mains from your Arduino is probably a good idea. There are plenty of opto's around that are easily interfaced to a mains AC supply as switched by your relay. "Encase" the opto-diode within a bridge rectifier circuit and connect the AC terminals of the bridge (via a capacitor and series resistor) to the AC.

The output of the opto is typically a transistor that will need a pull-up resistor to Vcc, maybe 10k ohm. Here's an opto that should work: -