This one is pretty easy. Corona virus typically causes a sudden onset of diarrhea, sometimes with vomiting. Anorexia and lethargy are also common. Infection can also frequently go unnoticed, especially in older dogs.
The incubation period is 1-3 days and the primary source of infection is feces. The virus can be shed in the feces of infected dogs for about 2 weeks. Most cases are mild and recover without treatment in a few days, although supportive and symptomatic care is indicated in more severe disease. Soft stool may persist for several weeks. There are tests available but they are not usually very useful, and mostly not needed because of the relatively short and benign course of the disease.
There is a vaccination available and it is often a part of the combination vaccines that include parvo, etc. However, the corona portion of the vaccine is no longer recommended as the efficacy is unknown and the disease is typically not severe anyway.