I think a good way to tell if you're in an echo-chamber is to express an opinion that is counter to the group and see if the community engages in active debate

I think a good way to tell if you're in an echo-chamber is to express an opinion that is counter to the group and see if the community engages in active debate or just tries to shun you. Not to the point of trolling, but just expressing an ordinary or commonly-held belief that is in contrast to the accepted group belief. Every group has them - aside from politics, consider e.g. expressing the idea that "Uber as an organisation is not immoral even if it has some bad employees" on HN right now. For the record, I think HN's actually pretty good at debating contrary opinions, a lot more so than most subreddits.