TimeRanges.length

var v = document.GetElementById("myVideo");

var buf = v.buffered;

var numRanges = buf.length;

if (buf.length == 1) {
  // Only one range
  if (buf.start(0) == 0 && buf.end(0) == v.duration) {
    // The one range starts at the beginning and ends at
    // the end of the video, so the whole thing is loaded
  }
}