How do I find out what application elements can be scripted with AppleScript?

AppleScript FAQ: How do I find out what application elements can be scripted with AppleScript?

Open up the AppleScript Editor (Applications > Utilities > AppleScript Editor), then go to File > Open Dictionary, and select the application you want to learn more about. For instance, I just wanted to learn more about scripting iTunes, so I chose iTunes from the dialog that was display.

Here’s what that AppleScript Editor “Dictionary’ dialog looks like: