There are two general categories of tests typically used for identifying viral infections: those based on viral genetic material and those based on host immune response.
The CDC page on MERS-CoV testing (also a coronavirus, just not the one that causes COVID-19) is an informative resource to start with (bold added and reformatted by me):
Molecular tests are ...
According to UpToDate:
At levels of therapeutic anticoagulation, VKAs (vitamin K antagonists) typically cause only mild prolongation (or do not prolong) the activated partial thromboplastin time (aPTT), and they do not prolong the anti-factor Xa activity level, a test typically used to measure the effect of heparin or direct oral factor Xa inhibitors
We have a more plausible reason for the reappearance in 1977 of H1N1. The thesis is that a live attenuated version of H1N1 (LAIV) was used by Chinese scientists who innoculated several 1000s military recruits. It likely was not attenuated correctly and reverted to a more virulent form in-vivo leading to the epidemic.
Between 1962 and 1973, almost 40,000 ...
According to a 1992 review the conclusion of release from a freezer is one obtained by elimination of alternatives:
Where was this virus for 27 years? The
possible explanations include preservation in a frozen state,
preservation in an animal reservoir, or retention in an
integrated, as yet undetected form in the genetic material of
a human or ...