I know! We see what we want to see, and I was struggling to keep that under control. I did know where to look, but I swear I did see a slight deviation. Since I corrected it, saw that it looked better, and the measurements confirmed it, I figured it had to have been there.
But yeah, the trick is to see the imperfection first, you're right.
Speaking of imperfection...
I've improved my cheshire dot considerably in the past few minutes. The secret? Don't try any of this when you or your eye is fatigued. It's a recipe for failure.
As for lighting up the cheshire, a bright flashlight is an awful idea, the glare induces even more eye fatigue (see above). Bright indirect light such as daylight provides a much clearer and glare-free illumination.
And if you're one of us over 50 types and you've got presbyopia, for goodness sake, use the correct strength reading glasses when peeping through the cheshire. I thought I could focus my eye at the distance from the cheshire to the secondary, but with a low powered (1.75) pair of reading glasses, the dot was smaller and the secondary donut was finer. I can easily discern centering defects that were imperceptible previously.
I'm coming around to the OPs way of looking at room for improvement. Getting the meatware optimized is just as important as the hardware.