you either have to start disconnecting elements or measure the earth resistance with a meter
from what your saying try disconnecting both wires to the top element. dont forget to note where the wires go. insulate the wires to make sure things cant touch. then turn on the lower element and see if it trips
if no it probably is just the top one
the only way of being certain is wait till it trips then disconnect. turn on again and see if it trips. or measure earth resistance with wires disconnected
Dave