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If comparing it to another eyepiece (such as a TeleVue), don't look in the center; look at or near the edge. Focus the stars there the best you can. Go back and forth through focus. Look for elongation in a radial manner on one side and a circumferential manner on the other side of focus. If you see this elongation, that is astigmatism. You're unlikely to see any above f/6.
Look also for field curvature: focus the center as sharply as possible. Then, without touching the focuser, look at the edge. Are the stars still perfectly in focus, or can you move the focuser slightly and sharpen the edge of field stars. If you have to move the focuser to tighten up the edge stars, this is field curvature. Even if it's a lot, you might simply focus on a star halfway from center to edge and let your eye's natural accommodation focus all the stars in the field.
Place a bright star just outside the field. Can you see any glow inside the field of view from the star outside the field? If you can, is it minor, or does it cause spikes or glare to appear inside the field even though the bright star is outside the field. This can be indicative of an eyepiece with a lot of scattered light in the field.
then, put the bright star in the center of the field and look at or near the edge. Is the background sky lighter out in the outer field than it is only 1/3 of the way out from center? That's also indicative of scattered light.
Then, look at a bright star-filled field of view. Is there any visible brightening of the edge of the field in the background? This is known as EOFB, Edge of field brightening, and it is better controlled in some eyepieces than others. I've seen some where the entire outer 50% of the field had a soft gray background while the center was a black background.
You can create a little graph with the comparisons between your eyepieces with None, a tiny bit (minimal), a fair amount (moderate), a lot, and Horrible (severe) as your grade points.
Last, just remember there is no perfect eyepiece, but some are more perfect than others.
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