How do I see if an authenticated network scan was successful or not?

You can check whether the authentication was successful to verify the success of your authenticated network scans. If one or two machines had a login failure, the scan status warning message will mention which exact IP addresses failed. If you want more details, you will need to check the info vulnerabilities in the scan result report or the Vulnerability Manager. Please activate the info severity level in the filter to access the information. 

Linux/Unix authentication

• SSH Login Successful for Authenticated Checks (HID-2-1-332007)
  Check if the login attempt was successful.
• SSH Login Failed for Authenticated Checks (HID-2-1-339493)
  Check if the login attempt failed.

Windows authentication

• SMB Login Successful For Authenticated Checks (HID-2-1-332096)
  Check if the login attempt was successful.
• SMB Login Failed For Authenticated Checks (HID-2-1-330558)
  Check if the login attempt failed.
• Check for SMB accessible registry (HID-2-1-33856)
  Check if the remote registry is accessible. If this HID is active, you will need to follow the steps mentioned in this article to enable the remote registry:
  What are the supported systems and requirements?