Signs that tell me "do not under any pretext be fooled into ordering coffee in this joint" include:
1 - pf out of the gruppa (of course)
2 - shiny bean bags or those big, inox cones that function as bean hoppers: these mostly mean that the coffee was burned beyond recognition by a large, native African/South American extorting multinational; and that aforementioned cremation of the coffee beans was effected more than a few months ago
3 - signs in French ("Hommes", "Femmes", "Menu du Marché" are specifically telling): these mean that we're in France, and coffee will be vile
4 - those gray-ish, old, battered machines that look as they haven't been cleaned properly since the last ice age. These are easily spotted from a few miles away by the pf's that are idling by its side
5 - silence, more specifically: the absence of any grinding noises when someone else orders coffee
6 - Illy logo's proudly on display on everything from the china to the napkins to the sugar bags to the spoons to the parasols: Illy cafés rarely get their coffee right. My guess would be that it has to do with telltale sign # 2 (cf. supra)
7 - Old, non-Italian people telling each other how good the coffee is
8 - little shot glasses filled with crème fraiche, on the little would-be silver tray that that the Illy cups are served upon