shopped pictures

There is no one particular way, it depends on exactly what was done to it. sometimes it's easy to spot...sometimes it takes carefull examination. Do you have a particular photograph in mind?

You can find some difference between a photoshopped image and an original image, the thing though is that most expert in Photoshop had develop this skill of mimic what's real that you can either tell if it's fake or not. Well, you can spot a photoshopped photo though by the skintone of the image, if you spotted that the skintone is finely enhance leaving not a single line or wrinkle and finely air-brushed image then that is a shopped image. But this is visible only to digital art and photo manipulation photography wherein the image is intended to be process to create a digital enhance form of artwork. But in natural photographic images, you really can't tell.