Since viruses cannot be grown in a lab dish (that is because viruses can only replicate inside a living cell), they cannot be cultured and multiplied like a bacteria sample.  So, the polymerase chain reaction (PCR) test was utilized and is the standard test to confirm COVID-19 infection today.

PCR involves a method to rapidly produce millions to billions of copies of a specific DNA sample sufficient enough for analysis, given that coronaviruses are only 80-220 nanometers in size.  COVID-19 is reported to be 120 nanometers in diameter.  (A nanometer = 1 billionth of a meter) Here is an electron micrograph image of a coronavirus.

A viral genome sequence was released to laboratories for analysis and the conclusion was it was "very closely related to members of a viral species termed severe acute respiratory syndrome (SARS)."  Based upon reliable confirmation of the PCR test using "virus isolates," the PCR test became the standard test for COVID-19.

The PCR test was initially validated by real viral isolates from the prior SARS (severe acute respiratory syndrome) outbreak of 2003.

However, researchers concede: "In the present case of 2019-nCoV (COVID-19), virus isolates or samples from infected patients have so far not become available to the international public health community." 

In other words, the scientific community did not have a complete virus to use to confirm COVID-19, only a "viral genome sequence," a snippet of genetic material "very closely related" to COVID-19. 

That was January 2020.  Fast forward to December 2020.  The FDA published a guidebook for PCR testing of COVID-19, marked "for emergency use only."  The title of the publication is: CDC 2019-Novel Coronavirus (2019-nCoV) Real-Time RT-PCR Diagnostic Panel, and was initially published in February, 2020, and then re-published in July.