Easy - jam a hose on the vac servo and inhale. But before doing so give the distributors' innards a liberal shot o' WD40 to free up the advance plate. If it's not perforated and the inside advance plate moves, you're rocking. There is a way to test the halls effect sensors in it but this one is a little unusual as it has 2 banks of them. Think all you need to do is hook up a multimeter to the wires and turn it by hand. The resistance values should go up and down (if I recall correctly that is) Man, that dizzy is crusty - where did you find it?