First, commit any crime until there is a yellow circle in your map that indicates you are commiting a crime.

Make sure there is no witnesses,if there is a witness, bribe or kill him/her.

After that, wait until the cop arrives. After that, they will shearch the area. Don't do anyting crazy if you want to keep unknown.

This will take some time, but after that, there will be a dollar mark on top of one policeman that indicates they are a corrupt cop.

You can then bribe him so you can do anything, like robbing a bank. But never kill a policeman or your bribe time is over.