When there is an actual sale, the value of the property is automatically updated to that sale price. If a property isn't sold for years, then the assessed value of the property will be changed based upon the more recent sale of properties around it.
So yes, it's what the county assessor thinks the value is worth. But each property owner also has the ability to appeal any increase in property value/tax, once every few years.
Anyhow, point being, it's all pretty public information.