I get that question every once in a while. Is it Photoshop? Or did Obama really name a Park Service boat Titan Uranus ? My usual response is to flip a coin (heads is photoshop) and add, "You can tell because pixels."
Well, yeah sometimes I can tell. But now there's a tool that is probably better: Foto Forensics. You need to read their tutorial but it looks like a useful site.
What I want is a site that can prove that this picture:
is, pixel for pixel, a direct copy of that photo of a kid with chocolate all over his face titled: I Don't Know Who Ate the Pudding Mom.