Shiny from GTS - Hack Check

[Antimorph]

Hello guys!

I'm here again because I love this community! Thank you all for helping me on my previous problems/issues.
Well... as some of you guys know, I've been playing pokemon since it's first release till the last days of pkmn Emerald. Then, they launched the very first Nintendo DS but my passion stayed with the game boy.
A month ago, I bought Alpha Sapphire. I've been loving it.
Today, I got a shiny from the GTS.

My question is: How can I tell if a pokemon is hacked? I know that are very special ways of creating hacked pokemon that will fool even Professor Oak, so I'm posting three pics of the shiny Sneasel I just randomly got on the GTS for a Cloyster (any gender, any level).

I would like you to help me to figure out if this pokemon is hacked or not. It's nature seems to fit on a "perfect pokemon". Nature, shiny... Well, let me know what you guys think! :D

Thank you again!

Other info:
- 1 IV (Sp. Def.)
- Memory girl at Lilycove says: The pokemon seems to have a good memory, but it doesn't seem to be able to remember...