How to check what file formats Google Docs app supports? I went to All apps in the Settings app, and then when looking at one, for example, Google Documents, I was gonna disable it but I got to looking at the information on the app. It says, some defaults set. Now, I do let some family members use my phone. I don't use Google Documents, never have. 

My question is how I can know which apps are using Google documents as a default?
Google Docs app can handle a large number of office text format files (.e.g MS Word .doc .docx .rtf ...). 

If you try to open such a file format using an app that can't read those format itself it sends the document to another app that supports this format. Which app is used is not app-specific, the association is saved for each document type system wide. Therefore there is no such thing like a setting for apps that are "using google documents as a default".
You can see see and mange the file associations in the system settings.

Also any app can make use of Google Docs, e.g. a mail program that uses an external program to view attachments. Therefore there is no way to detect apps that can make use of Google Docs. Therefore theoretically any installed app can make use for Google Docs. To narrow it down you can say that any app that handle files (from file-system, mail, chat, file sharing, ...) can make use of Google Docs.