Is there a definitive reference or introspection capability for Applescript?
Most apps that can be scripted with AppleScript will have a dictionary you can open with AppleScript Editor (File > Open Dictionary). The app's dictionary will then list details for each of the commands and classes that can be used with it in an AppleScript script.

Creating an AppleScript dictionary is up to the app's developer, so you may occasionally come across apps with poorly documented, incomplete or outdated dictionaries, but it's still usually a good place to start.