I have one, and when I use it with wet paper towels I usually feel a bit of something, though not very strong and not everybody feels it. Like biblo baggins said, testing to see if it functions is simply a matter of tasting the electrodes at the same time (on the end of your tongue) while plugged in to a working 9 volt battery - (Alkalike batteries last much longer then normal batteries, so I recommend them).
It's a set, square wave frequency and it doesn't have to go with specific frequencies. I'm not super tecghnical, so I can't really tell you how square wave makes more sense then sweep, but I'm heard the sweep don't work as well.