Not a bad advice to do it by measurments, but really if it's significant, it's not hard to notice, at least with headphones, I don't know bug at least 90% of the music I listen to have lead vocal and kick panned dead center. I notice if right away. Now for speakers, not sure how one would measure this. You may want to measure the accuracy of your balanced pot if you have one and see if center is really centered, but beside that, most of the imbalance you'll get is caused by your room snd speaker placement. In that case, maybe some have good advices, maybe measure one speaker at time snd see if there are level difference at listening position and compensate but still. I find ears quite accurate for that kind of things. If it sounds louder on one side and the voice is not at center, it probably isn't. Just don't use old Hendrix or Stones recordings for that.