First of all, let me tell you I had no idea. I'll tell you the story of my obtaining it to the best of my ability. If you still deem it fake after reading, well I apologize and offer to trade back for your Mawile.
This was the era of the shiny values, so I actually had to trade for this one. I had my own Honedge egg that I was trading the other guy, but since this guy was a bit of a douche (for lack of a better word), he wanted something in return. So he traded me his own Honedge for my egg, instead of trading me my egg back. To be honest, I wasn't too worried about a fake, since I had nothing to lose (only an egg). I never thought to check the date, which I guess can only be explained by his time traveling (I didn't hatch this one so I have no idea)? The pink smiley face and blue pentagon shouldn't indicate anything though, as I'm pretty sure those are normal.
But again, that's my rationale. I'm not sure how to check for legality, I'm pretty sure GTS and Wonder Trade only stop the 'mons with ribbons so that probably won't work. In any case, I apologize and offer to trade back if you wish.