I have a dialga, shiny and level 100. Seems pretty ok since dialga isn't shiny-locked in gen 6, but I got it from trade and it still has max happiness. Is it hacked?
Nothing explicitly points to it being hacked. There's not really a for-sure method to confirm whether it is or isn't so the best answer I can give is to suggest that you use your own judgement. It's just not definitive enough to tell for sure.