How do I know if it's safe to opt out of the Android Beta program without wiping my data? I recently joined the Android Beta program so I could update my Nexus 5x to the latest public release without having to wait for the staged roll-out.

Now I want to opt back out because I don't want to install a beta version.  When I go to the opt-out page, I see a warning that my user data will be wiped if I'm running a beta version:


  Unenroll device
  
  You are about to unenroll this device from the Android
  Beta Program. If your device is running a beta version of Android, it
  will be sent an OTA update that will wipe all data on the device and
  install the latest stable public version of Android.
  
  We strongly recommended that you back up all data before proceeding.
  You will lose all data that you have not backed up.


How do I confirm that I am running a public release?  And am I reading the language correctly that my user data will NOT be wiped, as long as I'm running a public release?
You aren't running a Public Release, or ?  In the first sentence you said you opted in to the Beta and are running "the latest public release"...You would first need to determine what Build you are running by going into Settings - About phone - under Build Number.

From the Android Beta site you linked in your post, under "How do I verify that I'm running a beta version of Android?":

1.  Navigate to Settings > About > Build number. If the build number starts with NP, you are running a beta version of Android N.

2.  Reboot your device. You will see a message informing you that your device is running a beta version of Android.

I would assume that if you are running a current Public Release that you would not have to worry about data loss, but as always, backup any files/docs/photos/music you would like to keep in case that happens before opting out of the Beta Program.

The lastest Public Release for Android 7.0 is NRD90S, and Android 6.0.1 is MTC20K according to what Google has posted here:  Google Nexus Factory Images