Actually a CT scan can only show the onset of pneumonia which can be caused by multiple different strains of virus including the coronavirus. But is currently being recommended by WHO as the fastest initial test for classification purposes.
To confirm if a person has ncovid-19 a blood culture test is needed but takes a few days for the results. Plus like you say some people do not present symptoms, that's why only a blood test can tell 100% if a person is infected.