The easiest non-destructive way is to find yourself a short, *clean* aluminium tube, take out the forks and headset, rub a little bit of scourer or scotch pad down inside the head tube and then press the short bit of aluminium to the interior wall of the headtube, making a good contact.

Break out a digital multimeter and look for a voltage, of more than 0.3v. See one, it's Ti, or a stainless steel/steel. No real big voltage difference, aluminium alloy.