I don't know how to tell legitimate movements from illegitimate ones, but the term of art "identity politics" refers to both. ID politics is a specific kind of political advocacy, and there are both good ID politics arguments and bad ones. You'd probably just have to investigate the claims they're making on a case by case basis.

But, I wasn't trying to interrogate whether defining yourself by outrage can be good in some instances, I was trying to point out that the term "ID politics" refers to things outside of defining yourself in relation to outrage. Maybe I just misinterpreted what you were saying, but I thought your comment unintentionally hinted that you were unaware the phrase is a specific term of art. There are many types of identity politics that aren't about outrage or opposition.