I don't really trust any Glassdoor reviews. I've always figured many positive reviews are fake, and the problem with negative reviews is that they're often written by disgruntled employees who are unable to reliably assess their own contributions to problematic situations.
The company I'm at -- the best company I've ever worked for, hands down -- has a few negative reviews from over the years that were written by employees who were fired for basically being awful, hostile, aggressive people who fought with everyone and refused to do work. These are the folks who are technically competent and think that gives them a pass for generally being an awful human being to work with. Being willing to fire people like this has resulted in this company having the happiest and most productive culture I've ever personally worked in, but it does lead to some pretty harsh reviews on Glassdoor. You're not going to get any of this context from the reviews.
The most informative reviews are the negative ones written by folks who left voluntarily and were not fired. In fact, these reviews, if they exist, can be a gold mine of valuable information. But it can sometimes be hard to pick these out, as folks who were fired sometimes lie about it and make it seem as if they chose to leave.
A generic tip when reading negative reviews is to look for specific details and concrete examples. The more details -- and the more specific they are -- the more likely the review is to have some basis in fact. This tip alone can filter out a lot of noise in both positive and negative reviews.
If you ever want to write a negative review, the corollary is that you should include specific, concrete examples to convince the reader that you're not just disgruntled. Avoid emotional language.
A negative review written with a calm, collected tone is also a sign that there might be something there.
It's also worth looking for themes that seem to be consistent across multiple negative reviews. As with Amazon reviews, it's often best to look at distributional properties of Glassdoor reviews rather than focus on specific reviews too much.