A pokemon granted from a 'fake' copy of an event-granter is totally and completely indistinguishable from one granted by a legit one. And even fakes made in other ways can be made completely legit-looking in every way, if the creator is careful.
Like the post above says, all you can do is look for details that are wrong, things that are set up the way a legit one could never be -- wrong stats, from the wrong place, etc. But of course someone knowledgeable who makes a fake can set all that up to look legit.