How can you tell the difference between plastic/polycarbonate lenses?

I'm not sure if you can really tell a difference. They are REALLY similar. I wanted to know the difference and I spoke with a sales rep at Replacement Lens Express. She put me through to the optician who explained it all to me, but in the end I just ended up going with plastic cause they were cheaper ($25 versus $53 for polycarbonate). I don't think they look much different, but apparently polycarbonates don't shatter as much and are good for athletes? I just don't know. If you're looking for actual professional advice though (and not me babbling!) email info@replacementlensexpress.com. They give good advice, and have great products, but don't really push them on you.