HR 15N is the Rockwell superficial hardness test with a brale indenter and a 15kg load.
You can measure carburizing case depth by mounting a metallographic sample from a coupon included with the load being carburized (or from a part) and polishing & etching the mount to visually examine the depth of carbon penetration under a metallographic microscope at 100 x magnification.
You can also measure by mounting a section from a part or a test coupon and running microhardness tests such as vickers or knoop from the edge in towards the center of the sample to determine when hardness drops off and estimate case depth that way.