You can "check" the headspace after you install an AR barrel.
I know of no way to "adjust" the headspace during the install.
There is only one way to install an AR barrel, and it either turns out right, or it doesn't.
In more than 99.9% of the installs, the dimension is correct when you measure it after the install.
If its not right, it isn't because of anything the installer did. In that case its a manufacturing issue.