Thats a long discussion. Technically he commanded a corps in Burma, of three Chinese infantry divisions. But, the Chinese placed so many restrictions on his 'command' he had little independendance. He also commanded the US 10th Army in the closing few days of the battle on Okinawa. Neither a clear test of his tactical & operational skills.
Prewar he had consistently been rated ahead of his peers in his performance evaluations. But, of course that was not in combat. Generally inter war US Army officers were judged on their ability to plan, organize, and execute a task. Every thing from whitewashing the curbs, to acting as the operations officers for a field exercise was judged in that context. If you could not run a command staff, or act as a staff officer competently you were headed for retirement as a 2d Lt.
This is something the Generals of WWII had in common. They were nearly all repeatedly rated above their peers in organizational ability. Even show boat Patton had at each command level from cavalry troop, to squadron, regiment & brigade been rated highly in his performance in staff positions, and as a commander running a HQ staff. Stillwells selection to command a division & then a corps in was based on a stack of annual & biannual performance evaluations that rated him among the top 5% of his peers. Of course Fredendal had similar top ratings for three decades, & we know how he fell apart under the pressure of ops in Tunisia January-February 1943.