Assuming your asking how to tell if your Pokemon is hacked, a way to do this is to put the Pokemon over the GTS.
The GTS is ran by Game Freak, and therefore they scan the Pokemon to make sure it's legitimate. They will happily let you put the Pokemon over the GTS, or they will not let you do it because it is illegitimate, or hacked.
I can tell this Mew is very sentimental, so just ask for an impossible trade like 'Zekrom Lv. 9 Under' or something like that to make sure that the Pokemon will not be traded. Of course, the Pokemon will only be up for like three seconds before you take it down.
Basically, if it can get on the GTS, it can get onto PokeBank.
Shiny Mews have been possible to get, clicking here you'll see.
Hope I helped. :)