Whats the best way of determining whether a wire is carrying AC or DC? Say its just some wire hanging out of a wall that I know nothing else about other than the fact that its carrying some kind of current. Perhaps this is a foolish question, but the rock band by a similar name is making my web searches difficult (even using advanced search features) 
. Thanks.
test it with a volt meter. Set your volt meter to read AC volts and set the voltage scale to 250 volts. And test it .It should be AC volts in your wall. not DC current.
Allan :
but how would it act if it was DC? My multimeter gets non-zero numbers regardless of whether I set it to AC or DC. Maybe thats where my problem is.
If we are talking about mains voltages it is ALL AC. There used to be some DC about 50 years ago but now ALL power coming out of the wall is AC.
The best way to test it is with a neon screwdriver. Touch the blade on the wire and touch the other end of the screwdriver with your thumb. It will light up. The neon light will be illuminated by light around both electrodes in the case of AC and on one side only in the case of DC. But you won't find DC.
If you want to use your meter, connect it up through a capacitor, with DC you will not get a reading, with DC you will.
Finally if you touch it an AC shock will feel like tingling and will throw you across the room where as a DC shock will paralyse your muscles and prevent you from releasing it, thus killing you.
Hook it up to an oscilliscope and either see the waveform or the static trace for ac or dc.
Hook it up to an oscilliscope
Problem with that is you have to connect the ground of the oscilloscope to one of the wires and if it is 240V AC and the ground gets connected to the live wire then BOOM, by by oscilloscope.