The only way you would know is if the holes in your perfectly vertical target shot at mostly a perpendicular angle were other than mostly perfectly round.
It is said, often, that a high twist rate is needed to stabilize a heavier, longer bullet. But this conclusion was reached by examining targets and impact marks. (not by actually seeing the round in flight)
The easiest way to see if you are achieving consistent, not necessarily perfectly stable, flight would be to shoot it well, and get a small group.