What applescript will a particular application accept?

I see a lot of applications that can respond to applescript messages such as this one.

tell application "Safari"
    return URL of fro开发者_如何学Pythonnt document as string
end tell

How can I query an application to find out what commands it will accept? Is there a tool for doing so?

In AppleScript Editor, do "Open Dictionary", select the application you are interested in, and you get a nice browser that shows the commands and their documentation (if the developers have chosen to provide any).