On first connection to a new site (typed without protocol), try https first
bugzilla.mozilla.org
bugzilla.mozilla.org
In short, there's no way to know if you're actually seeing the correct https version of the site.
It'll probably be no more than a month before everyone catches up.
That's anecdata, but to get good data one would have to try to fetch & compare results from both protocols for many websites - perhaps Google or the Internet Archive has done this?
The biggest annoyance for me in using https preferentially is that I often end up with multiple bookmarks for the "same" page, which differ only in their protocol - it would be nice if there were an auto-magic way to upgrade the old http bookmark to the https protocol.
You can see all the rulesets here: https://gitweb.torproject.org/https-everywhere.git/tree/src/...