Partially sacrifice one case, by leaving in flame until the flame color changes. Back off a couple tenths of a second or so and hammer through the entire batch. Flawless method, which doesn't require any special paint or thermometer, and will never risk ruining your brass.
Obviously an automated annealing machine of some kind is required for the best results here. I prefer the bench source unit.
Lots of details leading up to that point, but I'm assuming that's the part you were after.
Just about every "internet expert" on annealing will claim the brass is under-annealed... but I assure you I've vetted this with people much more qualified on the properties of metals and metal treatment than myself.