Hi Alice,
You have the right documentation : Previewing messages
You just need to click on the button Anti-spam analysis and get the complete list of scoring parameters for that message.
The rules are same as in V6. For example Text to Image ratio, not text content etc.
For a exhaustive list of rules spam Assassin (the software used for spam analysis in Campaign) , you can check this link from the official website : HowScoresAreAssigned - Spamassassin Wiki
Regards,
Adhiyan