Glassdoor does it for me.
Most reviews have a bias towards negativity (those with hard feelings will usually take the time to let everyone know), so if the reviews are extra negative it is a good sign to steer clear.
Reddit and HackerNews for more larger companies.
Also, recently discovered a Chinese language forum 1point3acrees where there is some real candid talk about the larger/FAANG companies and their offices around the world.