Nope, you don't know when you have the flu necessarily. Asymptomatic & mild flu is an established concept also.
What you thought was a cold may have been a mild flu, a strain similar to one you previously encountered.
Imagine a disease so catastrophic that you need a test to tell you you have it?
And cancer screenings have resumed though there is a backlog from spring. One of the reasons for the restrictions over past few months has been to ensure hospitals can continue with routine operations. In some other countries at the moment, to free up capacity as we did in spring, routine procedures have been cancelled.
The pandemic forced the suspension of services and reduced screening capacity
www.irishexaminer.com
In this scenario, think of the covid test as screening to try to detect it before you spread it to someone for which it could be 'catastrophic'.