It think x-ray were early tests and later they switched to ionization chambers with a La-140 radioactive source aka RaLa experiments[0]. Wiki says they conducte

It think x-ray were early tests and later they switched to ionization chambers with a La-140 radioactive source aka RaLa experiments[0]. Wiki says they conducted a total of 254 tests which is huge amount of work but hardly huge number of tests. Guessing if they had to determine the shape and composition of charges experimentally, it would've probably taken thousands of tests and a few more years. [0] - https://en.wikipedia.org/wiki/RaLa_Experiment https://en.wikipedia.org/wiki/RaLa_Experiment