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.

That first stroker engine I built had a distributor where the rotor started off on the right side of the post. As I applied vacuum the rotor would swing to the right and you could hear the spark jumping to the wrong post. The engine would start to misfire and run rough. That was a good learning experience for me.