The VA Tech Dendrology dept. has a nice page, I've put the link below. Also, NC State Univ. has a good number of information sheets on trees.

I will also type the tree into a search engine for info- but I check the source before I buy into the info. The .edu sites are usually my best bet. But several nurseries are also reliable sources.

Here is a link that might be useful: Va Tech information site

I've used the following website to help me study for a woody plants ID course.

http://www.hort.uconn.edu/Plants/index.html

- Audric

Also try Oregon State's . . .

Here is a link that might be useful: OSU Landscape Plants Images, Identification and Information

www.monrovia.com

I always do what creatrix suggested...simply type in your tree plus the word 'hardiness' into google and you'll be able to see several resources at a glance. I always look at three or more references just in case they vary widely.

And/or look at the Sunset Western Garden book.