Run your hand over the cover to see if it sheds any lint. If it doesn’t, it’s probably a good cover.
Check both ends of the cover to see if there is any fabric hanging over either end. Good quality covers are “beveled” and there should be no overhanging fabric.
Look for the seam in the roller cover. If you don’t see one, then its probably a good cover. If you see any gaps in the cover or loose backing at either end, that means the cover is of inferior quality.