My opinion only – there is a pretty strong, but not definite, correlation between goofy riders and lefties (and vice versa).
There is no gold-plated test to determine whether or not one is regular or goofy. I find pushing people to be pretty much useless.
I actually think that it matters a lot less than many people think. If learning to snowboard, wakeboard or similar where it’s not very practical to keep switching direction, then just pick one, stick with it and get on with it.
When I’m teaching people to snowboard, I always try to make sure their board is set up with a centred, symmetrical stance so that it’s easy to have a go both ways. Most of the initial phase of learning to snowboard is done in both directions anyway.