I think it's becoming a lost art ... but basically, you need to go by reputation. Pick well known sites that you trust, compare what they say about the subject, don't even base your opinion on just one random blog article or tweet / reddit / lemmy post.

For some, Wikipedia is trustworthy since it (usually) cites its sources and has a pretty good track record, while for others it's not to be trusted, cause anyone can edit it. In the end it's up to you what you trust. Another example: The CDC (in the US) can be considered trustworthy for health information, being an official government agency, but many also don't trust it as it has become more politicised and so, biased. Again, you decide what to trust, and always consult at least two trusted sources, more is better.

For product reviews, I simply don't pay much attention to the star rating, but instead, read the actual reviews, and sort them chronologically so I read the most recent ones. Check that they are actually reviewing the product / service you think they are, as there are ways to get good reviews then "switch" the product listing (amazon) and other similar tricks. Check if it seems plausible, level-headed, or if it's just someone being angry, or likely fake. Like I said, it's an art, not a science. Sometimes, you have to actually buy the product / service and judge for yourself, then compare your experience with the reviews, and you'll learn to tell the truthful reviews from the fake or unreliable.