A friend of mine swears by something similar to this, except you just have them stand there and when they're not suspecting anything you give them a good push in the back. Then whatever foot they extend forward to catch themselves is which side they might naturally favor being their downhill foot. Highly accurate, probably not. Entertaining, yes.
I'm a righty and ride goofy, but I basically figured it out through the initial trial and error that exists for everyone who is just starting to ride.
haha yes it is fun to watch but i think its more chance than accuracy that makes that method work
right handed or left handed doesnt make any difference in terms of goofy or regular im pretty sure