How to validate JSON with simplejson
Find the first non-whitespace character. If it's "<" then you have HTML.
Also, check the content type header and HTTP status code.
Occasionally I'm querying a server for JSON and receiving a 404 HTML page when the requested data is not available.  
Thus, I must have a check to ensure that the JSON I'm expecting, is actually json rather than HTML.  I'm accomplishing this now by checking for a string that I can expect to be in the HTML is contained in the response, but I think there has to be a better way to do this.