Is infection by covid 19 only detectable by test once symptoms start to show up or it can also be detected once someone is infected but symptoms have not shown up yet?

Both: It is just the fact that due to limited availability in most countries only patients with symptoms are tested, but the test can turn positive in patients with no to mild symptoms that have been exposed and tested for that reason. On the flip side patients with symptoms suggesting COVID may test initially negative, test my turn positive if retested.