The dizzy has *never* been out? Then I would hazzard a guess that it is probably OK.

But, if you have the valve cover off, then you probably should check. The marks are on the base of the dizzy shaft. One is on the gear, one is on the housing. This is all under the valve cover. You should be able to see them from either side of the gear on the cam. It will be tight, but it should be possible.

The bonus of doing this is that it will set a base timing that will allow the engine to start. Well, let's say, it take ignition timing out of the problem list.

You can approximate from where the rotor points relative to the cap, but adjustments in the dizzy position are enhanced at that point. i.e. A little movement of the shaft is multiplied by the length of the rotor. By lining up the marks at the gear end, it will be "closer".

Good Luck,
Michael