Convincing current employees may work on those that are isolated but if one works in a toxic environment they will likely know it. There was a time I would have semi-trusted sites like f'd company of long ago but those get infiltrated, sued and ultimately compromised by corporations that do not like people airing their dirty laundry and also gamified by their competitors.
If a new position turns out to be a crap-show I would just leave and not add it to my CV. If an individual manager is the issue, those come and go. I have found it very easy to make them explore new opportunities.
I've worked at places where employee surveys were done by the 3rd party to decide on winners. Companies that know they mistreat their employees won't bother and some companies will have management surprised when they find out how poorly their employees rated them on those surveys.
I don't think of it as really being a big positive, but it at least weeds out many of the worst.