Hi
Even if your active Sim is in the police career, you'll never be able to randomly go to another sim and arrest him. They can only arrest other sims during working hours, and in that case you're not able to watch it, you'll only get a message saying so.
There are some careers that you are allowed to see your sim working. If you have the Ambitions and Showtime Expansion packs, almost all of the professions that come with them require you (player) to be present in the working hours, this means that during working hours you'll need to make your sims to the chores it has to do all you'll see him doing them. You can read more about the careers here: http://www.carls-sims-3-guide.com/careers/
