When checking quality, you have to look closely at the video, and that can lead to your paying attention to artifacts that won't be noticed by the people watching your video, and possibly missing other artifacts that don't look as bad to you, but which are distracting when actually watching the video.
My method for identifying problems and knowing when I have fixed them is this:
First, I watch the compressed video straight through without pausing. I remember what distracts my attention from the video, and take notes.
I use these notes when working on fixing problems. I concentrate on those areas that distracted me the most. Although I might try to improve other artifacts that I see when working on problems, I make sure these artifacts are visible when playing at full speed.
When I think I have fixed the problems, I watch the fixed video again without pausing, trying to watch the way the expected audience would. If I don't see any problems watching like this, then the video quality should be acceptable.
