Carlos, Good question.

I use a Raytech infared digital thermometer I found in a pawnshop for $35.

You can use Tempilstiks at 600* and 650*. They are a hard marker that when at or above rated temperature they will mark the heated surface...too hot and it burns after it marks. This is done in low light. One thing you need to be careful of is heat soak. You do not want the heat to get to the bottom of the case. Heat it up and quench quickly. The worked hardness of the primer pocket is neccesary and softening that area can and will cause issues that could be catistrophic at the worst or cause loose primer pockets or the soft extraction lip would/could fail during extraction.

Tempilaq is another product made from the same company it is painted on prior to heating via a fingernail polish type brush. Our vendor stocks the sticks so I use that. These products are used in welding shops and where the temperature of metals needs to be quickly checked. I use them when heating bearings to shrink them on shafts where there is an interference fit. 200* +/-.