West Nile virus – Diagnosis and treatment – Mayo Clinic

Besides performing a physical exam, your doctor can confirm the presence of West Nile virus or a West Nile-related illness, such as meningitis or encephalitis, by performing one of the following tests: Lab tests. If you’re infected, a blood test may show a rising level of antibodies to the West Nile virus.

https://www.mayoclinic.org/diseases-conditions/west-nile-virus/diagnosis-treatment/drc-20350325