The Heartbleed bug is among the major security vulnerabilities we have seen in recent times. It’s one of those cases where precaution is the order of the day. You could manually check sites or use Chromebleed, an extension that tells you if the site you’re on was affected by the bug.
Chromebleed uses Filippo Valsorda’s little tool to test if the page was hit by Heartbleed and hasn’t issued a patch yet. You’re going to be safe on the bigger websites like Yahoo, but there’s a chance that some of the smaller sites haven’t yet patched their servers, so this little protection will help. If you do visit some such site, Chromebleed will throw a notification warning you, in which case it’s best to exit and notify the site’s developers to fix their issue.