How do you really find out about a company's working environment prior to accepting an offer, besides using Glassdoor?

In spite of efforts to find out, sometimes the full picture does not come to light until after you join the company. I was blindsided by a unique situation on a separate team that I could not have known about and that no one would have shared in an interview or otherwise. Disgruntled employees can appear out of nowhere and attempt to cause problems for any new employee who looks happy with their job. I chose to ignore it and discussed it with HR because it seemed like veiled harassment. Be cautious and don't let on too much if you like your job!

I would take Glassdoor reviews - both good and bad - with a large grain of salt.

Many of the bad reviews are written by disgruntled ex-employees. Think of some of the awful people you've worked with in the past - how much credence would you give their opinion?

I've also heard of managers making people write positive reviews or writing them themselves.