Just check their comment history for like 30 seconds. Everyone makes a joke that misses the mark every now and then, but the shittiest people usually constantly spew their nonsense.
The shit posters drive a wedge in a community and the people who don't like the "innocent" jokes leave. This makes fertile ground for ideological take over. Rinse and repeat in Fandoms, subreddits, message boards, etc.
Respectfully mention that it's kinda insensitive, kinda like, "hey man, don't you think that's kinda insensitive?" and you'll probably figure out fairly quickly which one is the answer. Additionally, if they legit don't know why it's insensitive, then you can educate them!
Just remember that tone can be difficult to convey on the internet, and sometimes you have to exaggerate the intended tone for it to be understood. Additionally, in my experience, asking the question from the other person's perspective ("don't you think that's kinda insensitive" vs "I think that's kinda insensitive") seems to help a lot.
My days on the Opie and Anthony reddit has given me good tone reading.
Fun racism and non fun racism. If it's ugly in tone they're racist. Remember Patrice or maybe Louis ck joking about how "jew" is both the normal term for Jewish people and the slur depending on the tone of how you say it. Reminds me of that but harder to tell with text.
The "extremist" part of Right wing "extremist" is meaningless. All reactionary politics are extreme, whether they take the active form of stochastic terror or they take the passive form of social murder (as defined by Engels in 1845, see below). The plausible deniability employed by reactionaries as camouflage among jokes is extremely transparent to anyone familiar enough with it.
This is why I try to give people the benefit of the doubt. I’d rather look naive than make uptight assumptions. Plus, I’m a reddit refugee, and I’ve HAD IT with pedants and the rampant holier-than-thou crap.