Simplest and most obvious way - do you have a button on the dashboard for activating the 4x4 system, and the corresponding green flashing light on the clocks?
If yes, then B-series. If no, then A-series.
Your car is late enough compared to the changeover period that it's almost certainly a B, but there are occasionally cars that lurk in dealer compounds and turn up on "wrong" plates when they get registered