I started looking for this feature to start an answer for: Tete-a-tete commonly known in English?
I'm pretty sure Dictionary.com used to have a little side bar where it said something like "most English speakers probably know this word." I don't know whether it was reliable or not, I haven't used it in years.
But, I can't find it anymore. Did they get rid of this feature? I can't find any articles announcing its retirement, either.