Look on the alternator itself for the red wire. If it goes into a plastic block made up of 2 red wires and a blue wire all together then it is a 55A, if it has separate red and blue wires that have their own nuts to connect it, then it is 70A.

Also, if you have a heated windscreen, you will have a 70A as standard (unless replaced by someone else who was too tight to buy the right one).