Recognising edited photos isn't always easy, but one surefire way to tell is to look at the light sources and the direction that they're coming from. Here's how to tell.
Bad or illogical lighting can makes it easier to tell if the focal point actually belongs in the photo or not, like the sectional above. The sectional is so bright during sunset that it's easy to tell that it's out of place, and the people are catching light from so many directions that you know the photo has been tweaked. If you suspect a photo of being touched up, flaws like this in lighting and exposure will give you clues.
We've gone over how to detect a Photoshopped portrait before, but AddictiveTips has some more tips for photographs in general at the link below.