I've wondered about that, too... there must be forensic photo analysis software made for that sort of thing that detects pixel irregularities, or something. Sometimes the edges/borders of elements are unnaturally jagged, or the lighting is clearly different in parts of the photo-- those can be tells.

I know that photos can contain metadata, and maybe date/time/location/whether it was made in photoshop/etc. information coded into the image could offer up some clues in certain cases (though that can be faked, too).

But yes, it's true, you can't really trust your eyes...