This is a bait question. You obviously believe this already, and for no good reason whatsoever.
Furthermore, the rhetoric of "nazi/facist/<whatever>" has been directed at every one of the last 3 Republican presidential candidates, the ones that the left now finds itself bemoaning aren't the Republican they ended up with.
I think the more pertinent question is "What level of proof would it take for you to realize that Trump is not a nazi or a facist or whatever the label is this week?" or perhaps "why should anyone take you even a little seriously with your worries about 'nazis' and such?"
I'm pretty sure the answer is "for him to turn into a liberal fantasy president."
This is what it looks like when the left does not get its way - when not only is a leftist not in the Presidency, but the left doesn't control Congress or the vast majority of state-level offices either. Let's not forget that - Trump, by himself, isn't really what's going on here. What's going on is that the Left has lost control of
almost everything and is now reduced to inventing moral panic in order to try to get the right to govern as if it were the left. The problem, of course, is that the press has totally discredited itself and what was left of that credibility was blown trying to drag Hillary Clinton over the finish line. The press is great at pretending that people are staring askance at Trump's first week in office because they are and washed-up celebrities are, but really that's not what's going on.
The tone of your OP is so **** arrogant it's almost unbelievable. "Well, we've concluded he's a nazi so he must be, what level of proof will it take for you to accept our righteous conclusion?"
If people like you are scared, he's probably doing something right. It is high time someone took a gigantic **** all over the left - and as for worries about his powers, Obama set those precedents when he couldn't understand that he was elected to curb Republican excess, not to turn the USA into liberal fantasy Euro v.2.0. That message was sent in 2010, and 2014 and he - and you - still didn't get it. Liberalism is a different worldview - it isn't "progress", it isn't "inevitable" and there is no "wrong side of history".
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It would be nice if both sides remembered this when their own guy is in power.
"Their own guy?"
How quickly we forget. Trump is not the Republican's guy. The party didn't want him. He was a joke, laughed at. He alienated huge portions of the Republican establishment. They were all convinced he couldn't win. I was convinced he couldn't win. I didn't even vote for him (though I would have, had I still been living in a swing state).
The Republicans have been put on notice as well - until academia and the media return to some semblance of balance, they are expected to stand up to them. It's the left presently out shooting people, smashing windows, making up false hate crimes, and acting like lawless bandits, and they have been since the election (on and off). In the last 2 years we've seen a total abandonment of any sense of consistency - if the election results had gone the other way we'd certainly not see such minimization from the press.
The failure of the Republicans to stand up to the national media has left them with Trump. The media is not a fourth arm of government that runs without checks and balances, entitled to anything it demands, and to be a left-wing platform deciding what is and isn't acceptable.