The term "Photoshopped" should really be replaced with "enhanced" in the common vanacular. As others have said, Photoshop is a tool, not an evil piece of image corrupting software. If there's any evil, it's in the intent of the Photoshop user, not the software.
On one hand, a really good digital artist can create a completely artificial image that looks real. On the other, all my RAW images (completely "natural" or shots of real, existing corals) are white balanced in Photoshop, where, ironically, I strive hard to make them look as they do in my tank (striving for realism, not manipulating for visual glory).
As IPT points out, there are common signs of bad digital manipulations. Light sourcing from different angles on different elements of the photo. A lack of shadow, or shadows conflicting or overlapping inappropriately. Bad seams between objects or at the edge of objects. A number of not so little circles where background texture doesn't quite match.