We basically already know how viral viruses are because they undergo mutations, and we can trace which strain people have and where they caught it from by seque

We basically already know how viral viruses are because they undergo mutations, and we can trace which strain people have and where they caught it from by sequencing their DNA (technically RNA) and looking for the virus's "ancestors". See for example https://www.eurekalert.org/pub_releases/2020-10/du-gt100120.php https://www.eurekalert.org/pub_releases/2020-10/du-gt100120.... Also you wouldn't mutate a virus with C14 or similar radiactive isotope, because that can't be encoded in its genetics, and such a virus would use any old Carbon atom (most likely normal C12) to replicate. Finally, mutating a virus and releasing it seems ethically dubious in the middle of a pandemic...