"Maintaining indoor relative humidity >40% will significantly reduce the infectivity of aerosolized virus."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3583861/
A hygrometer will check the relative humidity in a room. You can purchase a hygrometer for less than $10.
A word of caution. A whole house humidifier is more effective to maintain relative humidity then individual humidifiers. Individual humidifiers generally are not kept clean which can cause health risks . A whole house humidifier is easier to maintain for cleanliness.
