ArcGIS Pro 2.8 API Reference Guide
ViewingMode Property (MapView)
Example 

ArcGIS.Desktop.Mapping Namespace > MapView Class : ViewingMode Property
Gets the perspective of the view.
Syntax
public MapViewingMode ViewingMode {get;}
Public ReadOnly Property ViewingMode As MapViewingMode
Remarks
This property is used to determine whether the view is displaying a 2D map or 3D scene. A map view can be used to show maps which display 2D data or scenes which display 3D data. For a 3D view SceneLocal and SceneGlobal are valid view modes for the scene.
Example
Test if the view is 3D.
public bool IsView3D()
{
  //Get the active map view.
  var mapView = MapView.Active;
  if (mapView == null)
    return false;

  //Return whether the viewing mode is SceneLocal or SceneGlobal
  return mapView.ViewingMode == ArcGIS.Core.CIM.MapViewingMode.SceneLocal || mapView.ViewingMode == ArcGIS.Core.CIM.MapViewingMode.SceneGlobal;
}
Requirements

Target Platforms: Windows 10, Windows 8.1

See Also

Reference

MapView Class
MapView Members