Well-built applications (hopefully including most of Apple's own apps) should be self-documenting -- Apple recommends that developers include a preference manifest inside each application that describes the preference keys they use (including their names, data types, descriptions, reasonable default values, etc).
To view an app's manifest, right-click (or control-click) on the app, and select Show Package Contents; then open Contents/Resources, and look for a file or folder named after the app's bundle id with a ".manifest" suffix (e.g. Safari's is in a sub-bundle, so it's actually in Safari.app/Contents/Resources/com.apple.safari.manifest/Contents/Resources/com.apple.safari.manifest).
The manifest file is itself in .plist format, and may be a little hard to ready by hand, but if you look for the name values listed under
<key>pfm_name</key>, you should find what you're looking for.