How do I ensure that the macOS root account bug is fully fixed on my machine?
You can't at this point. The security patch only runs once and Apple hasn't released a new full point installer that guarantees you're patched.

What you can do is set a secure root password and watch for log in attempts and successes and disable sharing until Apple releases a full OS point release with no need for a Security patch to be put on top of one or more unpatched full point releases.

My answer here stands:


https://apple.stackexchange.com/a/307244/5472


If you can’t install the official patch or don't want to trust that it worked, then 

You don't want to disable root user on High Sierra only.

To secure your Mac, enable root with a long secure password.

We are not changing this at work until the next full point release is out for macOS which would likely be 10.13.2

I reason that since this was a race condition and Apple pushed the patch so quickly - we're pretty sure it's solid, but what if you got the patch before you upgraded to 10.13.1 and now it needs that patch a second time or what if the installer didn't really do it's job. Low chance either happened, but only you know how bad a breach would be on your computer with someone having total control.