I'm thinking they might look at your ratio of nofollow/dofollow links as normal websites don't have more than 5-10% nofollow, while aggressive SEOs tend to have 10%+.
CMSs like Wordpress would be in trouble, then, since Wordpress made the comment links no-follow years ago to make Google happy.
How about signs that the webmaster is closely monitoring their rankings and making regular changes based on the ranking changes they see? This seems to be the focus of a newly granted Google patent
THAT has been my tin foil hat theory for years. And the question of what we need to do is pretty simple: focus completely on your other traffic sources, and think of new ones often. Assume your Google traffic will disappear someday, and build accordingly. Once your sites are making enough without Google to at least scrape by, then your Google traffic income is just a nice bonus.
Because even if Google never knocks your site down in rankings, someday Google search will not be as dominant as it is now. Could be decades away, but it's gotta happen eventually.