To do the job yourself,...
you will need to have a telescopic gage for the interior of the choke, the reading is taken using a Mic or caliper.
This will allow you to take individual measurements from the muzzle downward.
You may also use an inside measuring spring caliper, as you withdrawl fro tube the legs will collapse but return to measurement which you take using a Mic or caliper.
Most choke mandfactureres will have a table of measurement for reference.