Layer.js

var pages = [doc pages];

for (var i = 0; i < pages.count(); i++){
  var page = pages[i];
  var artboards = [page artboards];
  
  for (var z = 0; z < artboards.count(); z++){
    var artboard = artboards[z];
    var layers = [artboard layers];
    
    for(var k = 0; k < layers.count(); k++){
      var layer = layers[k];
      
      if(layer.class() == "MSSymbolInstance"){
        //do something
        }
    }
  }
}