If you work with shared libraries and/or tinker around with ApplicationDomain you should go pick up the newly released SWFExplorer 0.7. It will tell you what classes are contained in a loaded SWF.

Info on it from Thibault on bytearray.org here and download it from here.

Being able to get a list or array of the classes in a SWF is a big help, and there really should be something in ApplicationDomain for that. If you agree, then please go vote for getDefinitions() and let Adobe know that you would like it added.