A quick and easy tip for spotting a fake "leak" is excess of information!
Looking back on the legitimate Brawl and Melee full roster leaks, they both have something very specific in common. They're just a list of names with no explanation. There's no "Sakurai wanted to add K. Rool, but there was rights issues" or "Sakurai added Balloon Fighter BECAUSE *reasons*". In fact, Melee's infamously listed "Roy", a character whose game had yet to be even announced in Japan, leading some to guess it meant Roy Koopa (or consider the roster false). The leaker didn't explain who Roy was or why he was added to Smash, because he doesn't know. The information is only going to leak out through content translators, people working on magazines or strategy guides, people who are just going to gain access to a list of names, possible description of design if they have access to pictures and nothing more.
For anyone to be close enough to know in-depth information as to who ALMOST got in, or WHY this character was chosen, they'd have to be a lead in the development team or higher up at Nintendo, who aren't going to be in the position to leak information. If anyone claiming to have sensitive information starts going into specific details into "Sakurai's process" for choosing or cutting someone it's pretty safe to say it's fake.