Glassdoor is absolutely filled to the brim with reviews clearly written by HR, marketing firms or by current employees who were told to write reviews.
If a review's only advice to management are: "Continue doing what you're doing!", "keep up the good work!" or "make sure our awesome culture isn't diluted as we grow!", then they're fake.
If a review's only cons are reframing the cons left my legitimate reviewers as sour grapes by someone who can't handle fast-paced environments, startups, or responsibilities that aren't clearly defined, they're fake.
But the biggest red flag is positive reviews left in succession over the period of a few days to a month.
Much like Amazon reviews, it seems that the only reviews you can glean worthwhile information from are the bad reviews.