Aprilia RS 125 testing the starter motor easy to do all you need is a good battery and 2 good heavy duty leads to connect to motor .
if your starter motor is fault try the links below to find parts
Buy Starter motors and starter bendix here in the UK
USA please check out here for Aprilia RS 125 parts
Attach leads Positive and negative correctly
clip positive to the Positive or main terminal
And touch negative lead to the body ( earth )
of the starter motor .
it can still spin bit not be powerful enough so remember that