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.
Actually, I set mine up so that under full load ( no or low vacuum) the rotor lines up perfectly with the cap terminal and under light load ( Vacuum advance in action ), the rotor lines up with about 1/2 the cap terminal. To the OP, Although Phasing the rotor to the cap is always a good thing to do / check and in that regard this is a good post, this did not solve my missing problem under load. My carb was about 5 jet sizes too rich ( although it had the "out of the box size jets in it ). Just so you keep and open mind to your problem.
