One difficulty is that fascism is a very modern phenomenon, in the sense that it stood out to 20C societies because they had known alternative forms of state power, whereas someone in 14C England, say, would not have known any other form of governance than the unchecked power of the State (strictly, the King) and in particular the use of force to compel behavior, which is of course the hallmark of fascism. It would be hard to recognize fascism unless you also knew what a democracy (say) or some other ruling mode was like.
The use of force to compel behavior is a defining feature of any state, not just fascist ones. Political theorists from Max Weber onward have described the state as the human community that “successfully claims the monopoly of the legitimate use of physical force within a given territory.” That applies equally to monarchies, democracies, empires, and totalitarian regimes.
The hallmarks of fascism include ultranationalism, mythic unity, supression of dissent, but go even further.
Instead of seeking stable control, fascism constructs an intense line of escape that it transforms into pure destruction and abolition, demonstrating a realized nihilism that goes beyond mere authoritarian management.
Fascist violence is driven by a psychological death drive—a desire not only to harm others but ultimately to destroy oneself. Under fascism, the government evolves into a suicidal state engages in a risky flirtation with its own self destruction.
Historically, this is evidenced by the Nazi rallying cry "long live death," the pivot of investments from production to pure destruction, and Hitler's 1945 order (Telegram 71) to destroy Germany's remaining infrastructure, effectively declaring that "if the war is lost, may the nation perish".
The suppression of dissent manifests as the accelerated destruction of state institutions, the severing of public assistance, and reducing the everyday presence of the federal government to masked armed agents in the streets, disappearing our neighbors and shooting civilians.
Above-and-beyons mere authoritarianksm, fascism exploits people's alienation, replacing valid grievances with a psychosis of total war and a nihilistic push toward self-destruction.
> whereas someone in 14C England, say, would not have known any other form of governance than the unchecked power of the State (strictly, the King)
The king's powers were not unchecked. People had human rights by custom and law (Magna Carter, for example), parliament controlled taxation, aristocrats had a great deal of power, the Church had a lot of power.
Nowhere near democracy, but a very different system from an unchecked dictatorship.
Edit to add: this was the system from which modern democracy slowly evolved.