Workflow:

•Determine one or two (or more) observer points.

•Create 3D sight lines between the observer(s) and the target.

•Lastly, determine the visibility between the observer and the target along the sight line.

Source: Esri Training - Line of Sight Analysis: https://www.esri.com/training/catalog/57fd7a465413c49402a2dd9b/performing-line-of-sight-analysis/