I guess it is mostly heuristics learned over a couple decades. I use an ad blocker (Wipr), so if there are problems loading the page, that is a flag. If it looks like an SEO churned out page with lots of links and poor sentence structure, another flag.

A lot of times, you do end up on Reddit and SO, but there are some good forums available if you dig deep, especially for car/HVAC/home maintenance related stuff. Sometimes you have to join a forum and post a question.