They are some of the most complex/problematic versions of this model of car, personally I would avoid owning one. Snow tires are a much better/cheaper solution. 
That said, you don't really need to "peel out" on dirt or gravel to test it, firm throttle on any unpaved surface will cause wheel slip which should trigger it (and it will be noticeable because it is an "after the fact" type 4wd so it will engage during slip)
Its far from very sophisticated since all it does is give power to the front wheels should the rear be spinning in comparison.
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