This site is very interesting!
It explains how Google looks at sites and rates spam sites.
The bottom line is that Google are using people to check that top sites are not spam sites.
Apparently this is rather similar to how Yahoo worked when they started. They may still use this method.
This site goes into exactly what rules are being used by the human checkers. The rules shown are about a year old. But people don’t change what they do much. So we expect they’ll pretty much continue to do rate them as shown on this site.