I think it's easier to tell exterior images are CG vs. interiors. I can only think of a handful of Viz companies doing exterior images where you really can't tell and they're usually doing a 3d model merged into a professional photo. Here's an exterior that's impossible to tell until you see the displaced grass in one of the close-up images -
http://www.evermotion.org/vbulletin/...ad.php?t=63778