There is no need to 100% inspect the surface hardness of the aluminum casting die inserts. The molten metal will do that for you. Molten metal will highlight the areas that are softer than the expected Rc 44 to Rc 50 range. Aluminum will solder to soft insert surfaces. Due to the physics of heat treatment the insert material at the bottom of the ribs is most likely to be out of specification. Unfortunately cleaning solder out of ribs is very difficult and is almost 100% likely to cause castings to stick in the die. Die casters such as Briggs and Stratton invented laminated dies for their deep fin lawn mower heads to enable creation of proper die insert hardness and draw polish access at the bottom of the fins.

As the die caster, you normally rely on the die shop to confirm the quality of the heat treat. Usually it is too late to fix improper heat treat after the insert is cut to final size. A set of hardness testing files and a visit to the die shop when the inserts are returned from heat treat are even more prudent as a success measure. The reputable die shops will welcome you.