I see things like roidapp, .chartboost, .mobvista, .ravesocial, etc. No idea what they are but eating up storage. How do I identify them as necessary?
These are folders apps use to store some or (less likely) all of their data. If you delete that folder, some app data associated with the folder will be gone.
In most cases you can safely delete that folder given that you uninstalled the app prior to deleting it.
The folders usually have the same name as the app they represent. The reason they have a dot at the beginning is so Android detects them as hidden and doesn't show them unless you tell it to do so.
You can just Google the folder names (without the dot) to find what app made them, what does it do and wether you want to keep it or not.