How can I trace which user account my kids dead phone had? My kid broke his phone. (Well actually, a bully classmate broke it..) It's all black, but probably runs. Not-rooted.

Problem: I've forgot his gmail-account. Can I grab the used account (not password) through adb or fastboot somehow?
Is usb-debugging enabled at all? Then you definitely can. That's one of the things my little tool Adebar does:

adb shell dumpsys account


gives you all the account information (which accounts exist on the device), including the Google one. No root required.