To make sure i understand you correctly, Are you saying if i know of someones test account, and im not connected to it at all, i can still use my production manager account developer token and query that test account and get the test data that is in there?
Concerning what im doing, i have a developer token (from my production account) and i currently use it everyday to query my live production google ads account. And that works all well. Recently i wanted to test some write api queries. I created a test account (not related to my production account at all). According to what you said, I should just swap my customerId key that i use for my production with my test account id? When i do that, and try to hit the GoogleAdsServiceClient.SearchStream endpoint, i get the following error
Exception thrown: 'Google.Ads.GoogleAds.V12.Errors.GoogleAdsException' in Google.Ads.GoogleAds.dll: 'Status(StatusCode="PermissionDenied", Detail="The caller does not have permission")'