And if the reddit post is a competitor paid smear, timed with some event, how could you possibly know? In that case, the glassdoor, linkedin, and reddit testimonial have the same weighted value of 0, as neither the quantity nor quality of reviews online can be trusted anymore.
If nothing else, looking at the history of the account (be it HN, Reddit, or something else) can give you a sense of legitimacy. Of course, even that could be faked or purchased, but if you have someone who seems to have a variety of other interests besides slagging companies, at least right now you can trust that it's a genuine review from a personal perspective.
Other folks might reply differently but my honest response is - I'm not going to do that. When I've switch jobs I've never had a shortage of offers so if I saw that post it'd raise a flag for me and I'd be less inclined to follow up with that company - other really positive factors might swing it back or not. This means I might end up missing a great opportunity but job searches are never exhaustive and we all just try to do the best we can to evaluate different positions with our limited resources.
I wouldn't - glassdoor and other moderated discussion boards allow for these smear campaigns to be weeded out but if their usage declines significantly then smear campaigns are going to get a lot more effective again.
I'm not saying the point I outlined above is the best outcome for the world, it's just the outcome for me and it's pretty regrettable that we're sliding back into it.
