I’m curious about this too, but I don’t think they’ll talk openly about it. For some hints, you could probably look at what data Google reCaptcha collects. The newest version (v3) doesn’t even require a user to click anything except a single button.

I bet Reddit and HN use timing, browsing time, clicks, and mouse movements, among other things to tell if a lot of visitors are just coming to the site to upvote a single story or are genuinely browsing the site before coming across something interesting.