Shoot an apple off their head by surprise and watch their reactions.

well, a more realistic test is watch them in situations of pressure, noise or danger. They are in a way reacting in opposite manners. Bartender in a busy club would definitely be a good job (though perhaps not that rewarding) for a SLI, but not for a SEI.
You may also ask them about their impressions, their feelings about random things. SLIs IME don't answer such requests, while SEIs find them natural, the can easily come up with and develop something pretty rich.