Floor it at the traffic lights in the wet, if it's a 4x4 you'll go forwards rapidly, if it's only running 2WD it won't

But remember that the Ford 4x4 system is 66% rear and 34% front torque split, this means that it will to some extent behave a little bit like a RWD car, that was their intention and why they spilt the torque like this. But generally you should be able to tell by flooring it in the wet as described, the 2WD won't have the traction in the wet, not by a mile.