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Everybody seems to have their own preferred method for deciding if someone is goofy or regular. So here's mine!

Go down some stairs normally, but hitting every tread. This depends on the fact that most people don't glide down stairs. They'll take two steps with a little pause. Left-right, left-right, left-right, to the bottom (or Right-left, right-left, right-left). Doing it the other way will generally feel pretty awkward. Whichever foot you lead with going downstairs, lead with on a snowboard.

I've never really understood putting new people in stance at 15 and 5. Whenever I teach people I put them on a twin tip (or as neutral a board as I can find) and put them at zero or in a natural/comfortable duck position (5 and -5). That way it doesn't matter if they're goofy or regular, and they can figure out which one they prefer just by changing directions.

Also, by having the angle close to perpendicular across the board, it's much more obvious that you control the board with your feet and not your upper body. Stand on your toes, you turn towards your toe edge. Stand on your heels, you turn towards your heel edge. Flex your feet in different directions, and the board swings out perpendicular to the direction of travel (i.e. brakes). What you do with your upper body is all about not falling down.

From a stop, both feet flexed uphill, relax one foot and that end will start to slide around and point downhill. Have them do that for a while, alternating feet and facing towards and away from the hill. Eventually get them doing the falling leaf routine if they don't pick that up on their own.
Then move on to making single turns, both directions, toe and heel edge starting.
Then move on to linked turns. Then they're done.

Then they can go change their bindings to 15 and 5 if they want.

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Simple Machines in Higher Dimensions