You could lick it, the tongue is pretty good at feeling that.
>>4 has the right idea
I would imagine some sort of crazy electricity measuring device could measure the electricity involved. Maybe you could also try turning on some speakers, grabbing the little 3.5mm cable that goes to them and seeing if the buzzing sound changes.
But I'd accept it as probably normal. I guess Mac laptops are pretty shitty quality, but both my MacBook and Powerbook had this issue when plugged in, and so does my Thinkpad X-series. Usually something's loose inside, and I'd just blame the fact your case\PSU\whatever else was made in China by slaves who don't give a shit.