There is a picture thread around, I guess you could use for comparison, or compare new shots to other pictures in your own personal albums.
Just make sure you are allowing the camera to focus or are tapping manual focus, and getting a variety of distances and light levels. Also try Camera+, as it can produce higher quality pics. This together should give you enough data to judge.
Bad things to look for are blotches in most photos, graininess or consistent lack focus, or inescapable purple haze all in your brain.