You would need to be familiar with photo editing techniques as well as limitations of camera equipment to be able to identify both what you would consider to be "photoshopped" as well as the signs of when it occurs. I'm fairly new to the game, but I can usually tell when an image has more dynamic range than is possible with camera equipment so maybe the sky's been swapped out (or at least the image was double exposed), or the light has changed directions or temperature, or if someone's skin is super smooth (that's an easy one; people have pores!), etc. You could maybe pick up a handful of telltale signs (HDR halos, for example), but it would only get you so far.
I've heard some people (National Geographic and some contests) that require submission of the original RAW files so that they can be compared to the result to identify images that have been altered beyond what they allow. This is necessary because people are becoming so good at photoshop that it is impossible to identify an altered image from looking at it.