If you do not mind earning Bad Cop points, use the following trick to rank up detective levels easily. Go to a place where there are lots of people, such as Central Park. Frisk as many people as you can, plant evidence, and arrest them. You will get one Bad Cop point if you arrest them correctly. Also, you will get three career points every time you do so. It should only take about a half hour at the most to rank up a level.
Use the following trick to get easy arrests without Bad Cop points. Go into a car dealership or a dojo and frisk all the customers inside. All the customers will have something for you to take as evidence. Arrest them for extra career points.
For easy arrests, run and tackle people, then punch them about three times. Get off them and someone near or that person will start to fight with you. Knock them out and arrest them.