Test. They administer test for detectives. You might also have to have demonstrated a proficiency for investigative works. Have an ability and keen understanding.. However ;if it is Civil Service they usually require testing for a pay grade increase like that.
There's a way around it. I believe you can study for being a detective (without ever having being a cop). Of course if you're a cop first you get a lot of experience, that you may not get if you study for detective.
Depends, some cops never become detectives. You must show independence, be alert to clues, ability to come up with the 'why's and 'how's from (the little) available info, know about the law, how to acquire clues, (and be a good cop), plus much more.