You can try imaging a very bright target with really short exposures. For example, get the very best focus you can attain and then shoot a minute of video of, say, Jupiter and run it through AutoStakkert. If the worst frames are this blurry and the best are much sharper, then you're fighting seeing issues. A one-minute exposure is way, way longer than typical seeing fluctuations, but if you take a bunch of images a fraction of a second long and you get some that are sharper, that should tell the tale.
If all of your video frames are much better than the 1-minute exposure (shoot one on the same target for comparison), then it might be tracking or guiding. Normally we see those as elongated stars, but it's also possible to have errors in RA and DEC that balance out enough to have stars that are reasonably round, but blurry.
You might also compare the results of lucky-imaging a bright star (not a planet) at the zenith with one near the horizon. At 0.5"/px, I would be astonished if there wasn't a visible difference, since you're looking through very different amounts of atmosphere, but it might give you a yardstick for seeing effects at your location.
As idclimber notes, sub-pixel imaging scales are pretty advanced, everything has to be spot-on to get reasonable results. Hoo-boy, I know that only too well.