How can the fields for Custom Reports be viewed?
Connector: Google Ads > Configuration > Custom Report
For Custom Reports, the fields are pulled directly from the Google Ads API based on the report name that is set.
By viewing the report name in Google's Documentation, from there you will be able to see the available fields. Here is an example for the campaign report: https://developers.google.com/google-ads/api/fields/v10/campaign.