I would assume that the Referer field of the request header would contain 'http://www.google.com' or something similar. Clicking on the question title for this question once already viewing it, then checking the request headers results in this:
Referer:http:
Clicking a link from a google search shows that the following is present in the request headers.
Referer:http:
EDIT: The search term for the google search that returned the results page that I clicked the link from was: "pointer-events: none opera"