It is easy enough to check, especially on a BB or Hemi since the distributor is right up front. Just drill a hole near the #1 post and shine your timing light down in there. If you have a Mityvac you can hook it up to the distributor and pull a vacuum on the can. As you pump up the vacuum you'll see the rotor swing clockwise.

I set up my distributors so the rotor is on the left side of the post with zero vacuum and then the vacuum advance will pull the rotor over to the right side of the post.

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