From the rise in use of propaganda, anti-press, anti intellectualism, nationalism, racial superiority, registry of others, and distrust and disdain of the corrupt government. Keep in mind that he is not only talking about Trump but the rise of the alt-right that promotes these ideals throughout the world. Everywhere these signs are visible means that a possible fascist takeover is not just a far off possibility, but quite likely.
To be honest I feel that Trump will be too busy dismantling anything in the way of profits and squashing the "losers" (which is another name for non wealthy, and trump critics) turning this nation into a plutocracy, and potentially laying the groundwork for a true aristocracy, to fight everyone and take supreme control as a fascist leader (he'll have plenty of help building a plutocracy, but few would help build a fascist state).
If the country suffers another major attack like 9/11 and he gets emergency powers though it might be another story.
Trump seems to fit into the mould of 1930s political figures such as Benito Mussolini. Mussolini who, like Trump, embraced rallies and bellicose, aggressive rhetoric as a way of arousing support. He also worked tirelessly to promote the interests of himself and the elite while blaming outside forces for the problems of the poor in his society, again similarly to Trump. Like Trump, he was also a deeply insecure and fragile personality who shamelessly attacked the press and could not tolerate criticism. Therefore the comparisons do seem apposite.
Yes, it is a fair comparison and more world leaders need to say so. There are far too many parallels between the rise of Fascism and Nazism in Europe and what's happening today in the U.S. and around the world. Unchecked power and corruption is what's happening. Trump and his cronies need to be watched carefully.
The opinions of Prince Charles regarding Trump should be kept to himself. This is an unfair comparison to link Trump to the 1930's. Prince Charles is not a political expert on U.S. matters and he should stay away from commenting on the President-Elect. The period of the 1930's was buried in a world war and this does not compare to Donald Trump.
Whenever one person disagrees with someone else politically, they compare the other person to Hitler. It has become such a cheap shot that it doesn't mean anything anymore. The people today know the warning signs of the past and totalitarian governments. They did not elect Trump for anything other than economic reasons.