There is a Chrome Extension called JSON View which validates your JSON and shows the error in a modal. Obviously you can use the awesome online tool JSON Lint to validate and format your JSON but this extension is pretty cool.
Update 18/06/13: Added reference to extension.
Chrome should do this out of the box I reckon.
Then when valid JSON we get this nice little collapse/expand node list.
It’s still not super smart though! Dutch?!? lol.