We don't.
There is a problem in American politics right now. Everyone is thinking in terms of “liberals vs. conservatives.” People think Donald Trump and his supporters are conservatives (spoiler: they're not). People think the folks protesting Trump are liberals (spoiler: they're not).
The divide in American politics is no longer liberal - conservative. That's old school, and it's disappointing that when it changed, so many otherwise intelligent people didn't notice.
Conservatives are not fascists. Ronald Reagan was not a fascist. George HW Bush was not a fascist.
Authoritarians are fascists. The divide in American politics is no longer liberal vs. conservative, it is authoritarian vs. democratic.
Donald Trump is not conservative. He is authoritarian. He is supported by many people who are otherwise liberal but pro-authoritarian. He is opposed by many conservatives who are anti-authoritarian.
Fascism is an authoritarian ideology. Specifically, it is an authoritarian ideology focused on nationalism, veneration of the military, xenophobia, doubletalk, rejection of individualism, categorization of dissent as treason, and rejection of empiricism, reason, and objective fact.
If you're still thinking in terms of liberal/conservative, you're not paying attention.