Most "asymptomatic" people will show symptoms within two weeks, so aggressive cluster/contact tracing works. South Korean local goveenment staff are following the credit card use, transit use, and other daily movements (CCTV logs) for every confirmed case, then issue two week self-isolation orders to those who shared a workplace, restaurant, cafe, church, or elevator with the confirmed case. Since most cases get a short lived fever or cough within two weeks, SK has been able to stop most transmission chains. I'm actually a bit confused about the scope of SK testing. It looks like everyone with symptoms is being tested, as well as quarantined health care workers and quarantined people from high risk groups, but I'm not sure whether asymptomatic people are tested at the end of their quarantine period.