I have one of those Stew-Mac checkers, just because it's handy and I didn't have a compass, but either one will work.
You shouldn't touch another magnet to your pole pieces, and don't get the Stew-Mac polarity checker too close to alnico magnets either. I hold it a few inches away from the pickup.
You can also just use a magnet with one side marked, and once again don't touch it to your poles.
I'd go with the Stew-Mac checker or a compass.