Duckduckgo does something similar. They use the rulesets provided by HTTPS-Everywhere. Not sure how Google is doing it. Maybe if a site redirects to https, and also has HSTS configured, it would be pretty safe to return HTTPS links instead of HTTP ones as search results.
Yesterday I noticed that DDG was no longer taking me to the https version of youtube and I'm fairly sure it did before. It looks like Google search results don't take you to https youtube either.
