SINner isn't a criminal. Criminal SIN means that.
How they do it depends on the game, as there are assumptions underlying how the game works that make a limited amount of sense.
In the RW, they run the license plate. This says who the owner is and if the owner is wanted or if the car is suspect in anything. If so, they make a felony stop.
Otherwise they get the license and registration from the driver and run it through the verification process that looks to see if it is valid, shows the picture of the driver (which should best match the guy behind the wheel) if there are any warrents or such out on him. I've seen some suggestions that some departments are using fingerprints to verify ID in the field.
http://www.out-law.com/page-7519http://72.14.253.104/search?q=cache:G0N60_...;cd=1&gl=usHow is is supposed to work in SR is unclear.
The cop should be able to read the SINs of all the occupants before stopping the car (RF) and run them through checks at a very high level. Plus he can check the car Id and see if it's or likely to be involved in anything that would provide a reasone for a stop. When he approaches the car I'd assume that the camera on his uniform records the driver and passengers and runs them through the database to match them (or fail to match them)to the SINs provided automatically without the officer doing anything.
I'd assume he can verify the ID through some form of biometrics, if he bothers.