Off.
If it's not got a button on the centre consol then it's accessed via the LH stalk buttons, along with fuel consumption, miles left in tank etc.
Pretty much all you have to do then is as you say: give it some hard acceleration from standstill on a loose surface.
If it goes like a bat out of hell then everything is working, loads of wheelspin, torque steer or grinding noises say find another
If you have a friend then they can watch the rear wheels also. They should throw gravel just as much as the fronts.
An '05 car may have the older generation Haldex which means the front wheel will spin a fraction (about 1/2 turn) before the rears kick in, 55 and later plate cars should all throw gravel from stationary as they changed the spec of the Haldex in '05 to have "instant traction".
If you have any doubts, either walk away or get it checked properly on ramps by a stealer / indi, for what that will cost the potential saving is huge if they find a problem
