If I am doing a request or subscription, how do i find out all of the fields available to be returned for a specific RIC?
If you're asking about real-time data interface, you can find the list of fields available for a given RIC using a Quote app in Eikon. Open a new Quote app, enter the RIC, then right click within the app and select Template - Display All Fields (or use the equivalent command in the app toolbar). This will give you a raw view of the real-time data record, which is a vector of field name/value pairs.
If you're asking about timeseries interface, you can discover the metadata for a given RIC using Formula Builder wizard in Eikon Excel. Or you can discover it using the API. See "Views and intervals" section in this tutorial.