To wrap up a coil in one of the 120v. wire is a good solution, but aditional circuitry is necessary.
A relay that works with 120v. is simple but not exactly cheap.
To use directly 120v. reducing voltage throw resistors and diodes is possible, but not safe.
To hack an ac adaptor is not cheap too.
But tell me one thing: do you need to know if the wires have exactly 120v.? Or the wires are connected to a 120v. socket and you just need to know if the voltage on it is high [on] (you already know is almost 120v. and don't need to confirm) or 0v.[off]?
In affirmative case, you can use another simple solution too:
to put a [small 120v. neon lamp+resistor in series] connected to 120v. wires, and use a [ldr+resistor] to switch 5v. when light is on (logic 1) to Arduino pin. A home made photocoupler (remember to put them together and isolate, inside some small closed box.