As someone who works in "STEM" (don't like the term but using for clarity): Ask their opinion on subjects that are not STEM. If someone is cool, the worst thing they'll say is along the lines of "I'm bad at geopolitics" or "I hated studying history in high school". Reactionaries however are filled with blind rage for humanities classes and will go on a rant about how they are useles useless and a waste of govermnent money to teach art at public universities and other shit.
The most dogshit wikipedia article that I have seen in a long while, and it is "locked from vandalism", incredible