Anyone who has been collecting a some time knows about these two sites as great resources for researching Hot Wheels lines and variations. For those that are new (or coming back) to collecting, when in doubt about your cars, consult either of these as a good place to start.
South Texas Diecast Collectors - The "bible" of Hot Wheels variations and wheel style guide. Also a local club with meetings and events.
Hot Wheels Wiki - Unofficial wiki for Hot Wheels variations. Nicely organized by year and series.