In the Terminal, you set the WebKitDeveloperExtras setting to true.
You can do this in Terminal like this:
for FileMaker 11:
defaults write com.filemaker.client.advanced WebKitDeveloperExtras -bool true
for FileMaker 12:
defaults write com.filemaker.client.advanced12 WebKitDeveloperExtras -bool true
for Xojo IDE:
defaults write com.xojo.xojo WebKitDeveloperExtras -bool true
or for other apps, simply use the their bundle identifier.
You may need to also enable debugging in Safari in the preferences.
And you can move the inspector to its own window to inspect the html structure even more easily as you see on the screenshot on the right side.