I wouldn't take everything on Glassdoor at face value, but if there are recurring negative themes in the reviews then that can be a red flag. I read them the same way I read reviews on Amazon, I look at the distribution of scores and then read a handful of each rating.
If there is a low distribution of 1 star ratings, then either the 1 star people are outliers or there is some sort of incentive for the higher ratings (either pressure from management to give positive reviews, or the company could just pay for fake reviews). Sometimes this is obvious, like a bunch of positive reviews that sound the same posted in a short timeframe, and sometimes it isn't. At the end of the day it is going to be a gut feeling.
I'm 100% in agreement. I disagree with those who say that negative reviews are more pure, and the 5-stars are HR / corporate drone-types. However, 'themes' and patterns are what I look for. Several times I've validated the experience of these negative themes, joining a place anyway or leaving a place and seeing confirmation. And just the same, if the positive ones say eye-rolling things like, "This place rewards the best, but they expect a lot from you. This is not for 9-5 whimps. Grrr." then I also see that as a pattern.
Before I was a contractor [ where reviews mean much less as you're not marrying your employer ], I'd take more of a Goldilocks approach to the reviews. The outliers (like a product review on Amazon) have to be smoothed out somewhat mentally.
If there is a low distribution of 1 star ratings, then either the 1 star people are outliers or there is some sort of incentive for the higher ratings (either pressure from management to give positive reviews, or the company could just pay for fake reviews). Sometimes this is obvious, like a bunch of positive reviews that sound the same posted in a short timeframe, and sometimes it isn't. At the end of the day it is going to be a gut feeling.