If they mention specific facts (extra bonus for verifiable ones), do not sound like ones written by poopheads or mass produced by competition, then it's a very strong red flag.
Things like unfavorable terms of service or checked-by-default, hard-to-notice checkboxes subscribing you to worthless additional services are easy to verify, even if you give zero trust to anonymous reviews.
You contrast "one star, it doesn't this obscure thing it didn't to do but otherwise it's great" versus "one star, it fell apart in three days, seriously" and get the idea.
It's a constant battle, altogether.
Hopefully, the Internet will continue to be deeper than the scamsters' ability to game. And hopefully Google will keep helping in this regard.
If you can do decent sentiment analysis though, you could lower the page rank granted by a link by sentiment of text in its proximity, while not lowering the rank of the page that contains the negative sentiment.
Genuinely interested, because the only reliable way I've found is to spend hours and hours reading through niche forums and reviews, and for every great product there are plenty of bad comments out there.