Sometimes its really easy to tell if its hacked or not.
For example..:
I was on Spp-WiFi the other day, and someone said he could trade me a legit Movie Darkrai for my Shiny Latios. So I asked him the ID and OT and everything. Everything sounded legit to me. So we switched fc and went on the WFC. Here is the part where I found out it was hacked. When I was checking out the Darkrai, yes it was held in a Cherish ball, everthing on the first page of the info was correct BUT when I checked out the other pages, the date said Jun 24, 2007 while it was supposed to be Jul 14, 2007. And it says met at lvl 50, but the Darkrai's level was 40, and it says Met from Pokemon Movie 16...(the newest movie is the tenth one =_=) So yeah, I said its hacked and the guy dced.