Re: check neck wall thickness on fired brass?
What is your specific concern? If you are going for extreme accuracy you can measue neck thickness for 2 reasons 1) separate by thickness for consistency 2) cull brass that is not uniform in thickness around the neck, non-uniform brass will cause some runout.
A brush in the neck will get any residue out but like Mikecr said, it's very thin.
The only time I measure neck thickness is when I'm reforming brass and have to turn the necks.