draw.on('drawstart',
  function(evt) {
    var sketch = evt.feature;
    var sketchGeom = sketch.getGeometry();
    // the 'change' event will will fired by mouse move or mouse click
    sketchGeom.on('change', function(evt) {
      // check the actual number of verticies against
      // my 'nodeCount' to see if the 'change' event
      // has created a vertex
    });
    sketchGeom.set('nodeCount', 1);  // add my own property to track the number of verticies
  },
  this);