Sorry, I might not be understanding where you're going with this. I initially thought you meant "you can know you're at risk of spreading COVID, because everyone is at risk of spreading COVID."

Certainly, anyone who receives a positive PCR test for COVID should quarantine ASAP. But if you're incubating COVID, but not infectious yet, the tests will return negative. A test only tells you whether you were infectious at the time of the test, so by the time you've seen the results, the information is already outdated.