I found this posted on another brick site and didn't belive it is had to try it.
OMG it works. I went to my local supermarket and started searching there selection of Lego Batman polybags.
The staff must have thought who is this guy and what's he doing. I must have stood there for 10 minutes.
The system is simple during the packaging process Lego add little stamp marks to the bottom edge. These sometimes look like air bubbles so be careful.
Using the system that Brickoflife.com posted I was able to get this little lot and I'm pleased to say the only shocking fact was it worked.