If you skate or surf then you already know which side to put forward. If you do not have any idea, then you are at an impass. People try to help you decide. I have taught thousands of people to snowboard and I honestly have never used a stance (regular vs goofy foot) determination routine that I trust.
Some say push somebody forward from behind.
Some say step forward.
Some say kick a ball.
Some say ...
I say trust your gut. Pick one and stick with it until you start moving around the mountain pretty good. Then work on fakie_
for the sake of fun, balance and muscle symmetry.
See also this thread on goofy vs regular