Detect if a transclude content has been given for a angularjs directive

Based on @Ilan's solution, you can use this simple $transclude function to know if there is transcluded content or not.

$transclude(function(clone){
    if(clone.length){
        scope.hasTranscluded = true;
    }
});

Plnkr demonstrating this approach with ng-if to set default content if nothing to transclude: http://plnkr.co/hHr0aoSktqZYKoiFMzE6

After release of Angular v1.5 with multi-slot transclusion it's even simpler. For example you have used component instead of directive and don't have access to link or compile functions. Yet you have access to $transclude service. So you can check presence of content with 'official' method:

app.component('myTransclude', {
  transclude: {
    'slot': '?transcludeSlot'
  },
  controller: function ($transclude) {
    this.transcludePresent = function() {
      return $transclude.isSlotFilled('slot');
    };
  }
})

with template like this:

<div class="progress" ng-class="{'with-label': withLabel}">
    <div class="label"><span ng-transclude="slot"></span>
    <div class="other">...</div>
</div>

Here is a plunker: http://plnkr.co/edit/ednJwiceWD5vS0orewKW?p=preview

You can find the transcluded element inside the linking function and check it's contents:

Directive:

app.directive('progressbar', function(){
  return {
    scope: {},
    transclude: true,
    templateUrl: "progressbar.html",
    link: function(scope,elm){
      var transcluded = elm.find('span').contents();
      scope.withLabel = transcluded.length > 0; // true or false
    }
  }
})

Template:

<div class="progress" ng-class="{'with-label': withLabel}">
    <div class="label"><span ng-transclude></span>
    <div class="other">...</div>
</div>

---

You can also create your custom transclusion directive like so:

app.directive('myTransclude', function(){

  return {
    link: function(scope, elm, attrs, ctrl, $transclude){
      $transclude(function(clone){

        // Do something with this:
        // if(clone.length > 0) ...

        elm.empty();
        elm.append(clone);
      })
    }
  }
})