macOS Sierra does quite a few things differently with the App Store, including the lack of a debug menu option and the lack of a com.apple.storeagent.plist which is where folks used to cull that information for reporting purposes.

After asking in the #jamfnation channel on the MacAdmins Slack a few of us came up with the following alternative to get that information:

/usr/libexec/PlistBuddy -c "Print :PrimaryAccount:0:1:identifier" /Users/$username/Library/Preferences/com.apple.commerce.plist

I created an Extension Attribute out of the above here. Detailed blog post forthcoming.