CEP: How to find out which layers are active

You can try this function. 

alert (getSelectedLayersIdx());
// by paul mr;
function getSelectedLayersIdx(){ 
      var selectedLayers = new Array; 
      var ref = new ActionReference(); 
      ref.putEnumerated( charIDToTypeID("Dcmn"), charIDToTypeID("Ordn"), charIDToTypeID("Trgt") ); 
      var desc = executeActionGet(ref); 
      if( desc.hasKey( stringIDToTypeID( 'targetLayers' ) ) ){ 
         desc = desc.getList( stringIDToTypeID( 'targetLayers' )); 
          var c = desc.count 
          var selectedLayers = new Array(); 
          for(var i=0;i<c;i++){ 
            try{ 
               activeDocument.backgroundLayer; 
               selectedLayers.push(  desc.getReference( i ).getIndex() ); 
            }catch(e){ 
               selectedLayers.push(  desc.getReference( i ).getIndex()+1 ); 
            } 
          } 
       }else{ 
         var ref = new ActionReference(); 
         ref.putProperty( charIDToTypeID("Prpr") , charIDToTypeID( "ItmI" )); 
         ref.putEnumerated( charIDToTypeID("Lyr "), charIDToTypeID("Ordn"), charIDToTypeID("Trgt") ); 
         try{ 
            activeDocument.backgroundLayer; 
            selectedLayers.push( executeActionGet(ref).getInteger(charIDToTypeID( "ItmI" ))-1); 
         }catch(e){ 
            selectedLayers.push( executeActionGet(ref).getInteger(charIDToTypeID( "ItmI" ))); 
         } 
      } 
      return selectedLayers; 
};