CheckForExistingSyles.js

var sketch = context.api();
  var doc = sketch.selectedDocument;

  var sharedStyles = doc.sketchObject.documentData().layerStyles();
  var numberOfSharedStyles = Number(sharedStyles.numberOfSharedStyles());

  //create array to hold existing styles for reference later
  var existingStyles = [];

  //if there are exisitng styles push them to array
  if(numberOfSharedStyles > 0){
    for (var i = 0; i < sharedStyles.numberOfSharedStyles(); i++){
      layerStyle = sharedStyles.objects().objectAtIndex(i);
      var styleName = String(layerStyle.name());
      existingStyles.push(styleName);
    }
  }