Here’s some Jscript that shows how to find clusters hidden by ToggleVisibility.

Note that clusters can also be hidden by adding a visibility property (Property > Polygon Cluster Visibility).
Finding those would be a separate, but more straightforward, task

// Get a polygon cluster
//var oCluster = Dictionary.GetObject( "cube.polymsh.cls.Polygon" );
var oCluster = Selection(0);

// Get array of polygon indices
var a = oCluster.Elements.Array.toArray();

// Get [basically undocumented] invisible polygon cluster
var oGeometry = Dictionary.GetObject( "cube" ).ActivePrimitive.Geometry;
var l_polyVisCluster = oGeometry.Clusters( siInvisiblePolygonsClusterName );


// Check if a polygon in our cluster is hidden
// It is possible for a poly to be in multiple clusters
// Otherwise we could assume that if a[0] is hidden, 
// then the whole cluster is hidden

for ( var i = 0; i < a.length; i++ )
{

                if ( l_polyVisCluster.Elements.FindIndex( a[i] ) == -1 )
                {
                                LogMessage( "poly[" + a[i] + "] : Visible" );
                }
                else
                {
                                LogMessage( "poly[" + a[i] + "] : Hidden" );
                }
}