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. Both IMO tend to have a good kinetic memory - ability to reproduce something they have previously performed - however SEIs are sensitive to the emotional context: for example the prospect of performing an operation of irreversible results (risky, requiring precision) will unbalance the SEI, though such subjective augmentations of the state of things will generally not affect a SLI.

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.

This is just what comes to my mind, there surely are many other methods to differentiate these two types (I haven't read the thread).