Coming from a different angle, could you use a small power transformer (like a mobile phone charger) to step the voltage down to 5V, then just detect it on a digital pin?
You'd have to hack up the power pack if you don't have a standard outlet, and I guess you'd have to be confident in your ability to work with high voltages.
What trialex proposes seems like a better solution. At least you would be sure it worked without all the hassle of figuring out the amound of windings needed to get a readable voltage. 
Just hack open a 120VAC/5VDC transformer and hook the primary coil up in series with the equipment on the power outlet (break one 120V wire and hook the two ends up to the two pins on the primary coil of the transformer).
But like he sais, working with mains voltage isn't the thing for you unless you know what you are doing! It could very well kill you, and if you survive but don't use the correct equipment or thick/robust enough wires it might burn down your house (again as likely killing you as not). To dampen your christmas DIY spirit even more.. if your house burns down due to you tampering with mains wireing, your insurance company will just say "FU asshat, we ain't paying because you just had to hack apart your mains wire!". That would kinda suck... -_-'
Kids, trialex has a great idea, but the moral here is: play with fire (or rather high voltage electricity) and you could get burned! So unless you are confident in your abilities.. just don't. 