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.)
Yuck, generally military history and military hardware interest are pretty major red flags to me. squints at news megathread that I regularly post on
It's one of those hobbies where everyone's a communist or a fash with very little in between
there's a lot of liberal military nerds nowadays since the SMO, an interest in military history nowadays is more indicative of just having a strong political stance in general now more than being linked to a tendency.
A "strong" Liberal stance is just fascism. Fascism = Distilled essence of Liberalism
In many cases it is
red flag.
British flags