Adjusting certain values and color channels(among other tools) in photoshop you can usually spot artifacts that would reveal forgery. It's extremely difficult to edit a photo where the added layers will change in the same exact way as the rest of the image.

If this image was fake you'd likely see some strange edges or discoloring that would really stand out in the edited areas. Which in this case would most likely be the faces in the photo.

Since these images have the same effect evenly distributed across the entire photo, with no weird portions standing out in anyway, it's very likely the image is real. Though it's hard to know what kind of image faking technology exists. This is a very simple and common way of spotting photoshopped images.