For me the easiest tell is the up front, unprompted, and unsolicited declaration of nonpoliticalness. When someone takes the time and expends the breath to announce how nonpolitical they are, what follows is almost always a rant about how everything/everyone else is too political these days, and that of course leads into something between status quo advocacy and outright reactionary/regressive sentiments for some fabled time before those wicked politics were visible to the nonpolitical ranter. 
People that are hostile to service workers. Some just want to take some ideological stand against tipping when the service worker doesn't really have a choice and needs those tips to survive in the current unjust system in a way where ideological purity gestures toward that service worker just look like being a greedy and sanctimonious asshole. The worst of such people will actually declare, shamelessly, that they believe that service workers don't deserve a living wage. The implications of that are 
worthy.
I may get shit for this, but I'll say it anyway: this hair and beard combo, seen on living people. 
I have yet to meet anyone in person with that look that wasn't a chud.
(If one of you is a comrade with that look, I am sorry in advance for the prejudice and if I ever meet you in person I will atone by buying you a drink or something.)
Thinking crime has gotten worse, it's an absolute TV talking point that everyone repeats nowadays. If you try to bring up empirical evidence of crime decreasing every decade they just respond with some anecdotal bullshit they probably made up. It infuriates me and I wanna shout their face, "facts don't care about your feelings".
I always reply in agreement then follow up about how imperial plundering of the global south has been increasing with the turbocharging of neoliberalism and all the criminal leaders and functionaries of the the US and its allies should be put in prison while letting everyone else out
I've gone the similar route and just acknowledge that all our business and political leaders are criminals, makes sense the rest of us are criminals too.
Bonus flags if they mention Chicago in this context.
Bonus points if they do this while living in Chicago or surrounding areas
Real John Kass hours
it's definitely been said elsewhere in this thread but my default assumption when meeting people from the suburbs is that they're chuds and it is correct like a solid 80% of the time
"REEEEEE CHICAGO IS GANGLAND" -
oh if they have a specific Democrat Run City in some other part of the country that they hate there's no ifs ands or buts, they're a fascist
This is a red flag, but I'd say it's probably 50/50 on whether someone will dig in when confronted with the reality of the situation vs. listen.
The big issue is getting people to not just listen and nod along, but remember that next time they see a crime story instead of defaulting back to the media narrative.
Both the Conservative CSU and the fascist AfD are running "tough on crime" campaigns here... bruh, even in bigger cities like Nürnberg or München you could leave the door open all day and odds are nothing would happen to your stuff. It's entirely "young men speaking arabic on the streets - this is very scary!" based, even if this overt forms of scaremongering have passed onto more subtle ones.
Bavaria has like 5 cops per resident, wtf are these people going on about?
They look at the United States with hearts in their eyes, like in old western cartoons.
every day my fucking coworker tells everyone about some new violent crime that happened somewhere in the greater metro area because he has an app on his phone that apparently gives push notifications for this stuff. i'm thinkin like "dude why do you have that you are driving yourself insane"
yes he's a homeowner who commutes from the suburbs, how could you tell?
We have local Twitter accounts and it's just some guy with no life posting everything he hears on the scanner.