My French is way rusty these days, but I accidentally picked up a great piece of slang while watching Le Corbeau last weekend. In at least one of the hate letters, Le Corbeau writes: "J'ai l'oeil américain et je dirai tout." The Criterion subtitles translate this straightforwardly as "I see all and will tell all"; however, not having heard of an "American eye" before, I was curious about how common an expression this is.
Unfortunately for me, I still don't have an answer for that. But in its online dictionnaire d'argot et du français familier, languefrançaise.net does provide the following helpful definitions for the term: "espion habile, observateur, oeil vif, exercé; oeil scrutateur, oeil séducteur; voir clair (fig.); être observateur, bon observateur; découvrir qqun, qqchose du premier coup d'oeil (particulièrement une ou des femmes)." That, my friends, is the slang of the week.