I dont know the exact procedure but I know for example some brands will reach out to ex workers or colleagues to have a clear picture.
That contrasted with workers in the same field can give a good view. Plus social engineering to dig up.
I know because of this, some creators have strengthen their NDAs to ridiculous points.
One example, if someone made something for Jenna, like a short video, they are mandated to give up all authorship of it. Down to graphics or art.
In plain words. If you ever worked for her, no you didn't, she can claim she did everything. (Thats why she also pays dirt)