The typical method to determine the input energy is to process the actual module to be soldered through an oven or other reflow equipment attached to a thermal profiler with thermocouples at the solder location. During this pre-processing, no solder is used.
A customer recently introduced me to another method for determining the input energy required to reflow a solder joint. They used differential scanning calorimetry (DSC). With this process, they inserted their entire module (solder and all) into DSC equipment. When a phase change was detected due to solder melting, a peak appeared, providing the applied temperature. This heat input can later be applied into a reflow profile used to process a high volume of samples.
I do not believe this tactic is as controlled as using a thermal profiler, however it provides another option for solder profiling for those of you with DSC capability. Please comment if you have tried this with success or failure. I'd love to hear from you.