Mark - There are a couple of methods available - measuring and marking the damper is one - sometimes hard to get in there after everything is together. But sometimes its the only way. Second is to use a "dial back to zero" style of timing light. Mine is an old Sears unit and seems to work well, many do - but I have heard compaints about the accuracy of others. Its a pretty useful instrument if you are going to set-up a distributor right there in your driveway. Better to read the directions for the unit you get than listen to me! But it will let you set the initial timing, measure the centrifugal advance and the total timing. At idle, you can see your initial - then plug in the vacuum can and measure the increase directly.