Perhaps the following solutions are possible:

- Hardware fingerprinting with the canvas-tag ( http://cseweb.ucsd.edu/~hovav/dist/canvas.pdf )

- Somehow check which assets are preferred downloads:

  rel="apple-touch-icon" sizes="144x144"  
  rel="apple-touch-icon" sizes="114x114"  
  rel="apple-touch-icon" sizes="72x72"