Safari extension to check if you're currently logged-in to Google/Facebook/etc.? Is there a Safari extension anywhere that shows a banner (or something equivalent) whenever you're logged-in to Google/Facebook/etc.? 

I rarely sign into Gmail from the web, and when I do I want to make sure I sign out as soon as I'm finished doing whatever I'm doing. I often forget though, and only notice days later that I've been logged in that whole time. 

An extension that gives a clear "warning" when you're logged-in would be fantastic, and solve my problem, but my searches thus far have turned up nothing.
I don't know of an extension that will warn you about remaining logged into Facebook or Google accounts. However, since your goal is to block web trackers from services like these, a better solution is to install an extension that blocks web tracking bugs. I personally use and recommend Ghostery. Ghostery works with Safari, plus Chrome, Firefox and IE, and it blocks Facebook and Google trackers, plus a slew of others as well. With a plugin like this you can also safely remain logged into your Google and Facebook accounts as well.