I think they look at 5 things..
1) Length of the text on the page
2) If it is 100% unique
3) If it includes images, or videos
4) When was the last time it was changed
5) your bounce rate
Its not tough for Google to just implement a grammar and spelling check. It will also check for duplicate content. For those who say its too hard you are very wrong. Look at Microsoft word it can tell grammar and spelling errors. Obviously Google has no problem checking for duplicate content it can just query its database.
Duplicate content again, I bet you do not even know what it is.