WEP can be 64, 128 , 152 or 256 bit.

It depends if you are anyalising captured packets or just "sniffing" around picking up networks. WPA assigns a dynamic key where as WEP is a static key, hence the well known security flaw in WEP, if you can capture the relevant packets the hash will always be the same with WEP.

WPA on the otherhand only has a valid key per "session" so if you capture the relevant packets but yet the hash constantly changes you know the network has WPA.

Not a detailed discription i know but more of a "in a nut shell" discription, obviously it is a lot more in depth than this!

Take a look at the advanced section here: http://www.tamos.com/htmlhelp/commwifi/aboutcvwifi.htm