I could probably tell you if I knew what a sub-inspector of police did. I would say just like any other ranking/of a police force you would have to take training,classes and work up through the ranks while learning on the job. In other words take both physical and mental tests in order to advance.then I would also say college/to learn public relations and speaking.maybe even military service.if I was checking into this I would talk to a sub-inspector and find out what they had to do in order to advance to that position/rank. Who better to tell you than someone that is already there in that position.
