Experts recon 2 to 3 days before symptoms show. I guess it's like most virus. The period were most infectious is first 5 days.
As I understand it it's 2 different test millertop. One to detect people who have already have it and one that tests for people who have had it.
The test for who have it according to news takes 24-48 hours for the result so imo pointless for the frontline workers who are still working. They could easily catch it even before the results come back.
The better test would be for those who have had it. They could carry on working plus they wouldn't need the PPE so would release more for others.