Well, 1st of all, start it up in 2WD, and change the selector to 4WD drive (and back), and see if the indicator on the dash shows "4WD", then "2WD"...
But obviously, the next step/easiest way would be to take it somewhere (like a muddy hill or slope) where 4WD would be helpful, engage it, & see what happens
I did these things before agreeing to buy my '011, and all went well, so I bought it asap, and thankfully, there has only been 2 occasions where I needed it...
Worst case, you could take it a gearhead/mechanic that works on 4WD's and get them to check it out for you....