Knowing what breeds might have gone into the cat is a tremendous help. I keep a couple of breed books around, and I've learned many distinctive signs; it's amazing how many mixed breeds nonetheless have distinct breed indicators.
Currently I have cats who look like a Norwegian Forest Cat, Maine Coon, and Tiffany, even though it's equally obvious they are mixed breeds. But this has been a big help in understanding their behavior and their needs.
Here's a breed resource on line:http://www.cfainc.org/
And, of course, each breed has at least one Fancier site.