Different size and color.
A PCI slot is normally white and you can see a few in this image.
http://www.hwlux.de/Images/Epox/8RDA3/146-4636_IMG.JPG
and I believe (I don't know because I've never seen one) that PCI-E is black, but there is some variation among the manufacturers in terms of color.
http://www.hwupgrade.it/articoli/108...ci_express.jpg
I don't know much about PCI-E, I stopped being to afford and care about the latest and greatest mouse trap years ago. The best way to find out is to read your motherboards manual or go to the motherboard manufacturers website and download it.