How to test if a string is JSON or not?

I have a simple AJAX call, and the server will return either a JSON string with useful data or an error message string produced by the PHP function mysql_error(). How can I test whether this data is a JSON string or the error message.

It would be nice to use a function called isJSON just like you can use the function instanceof to test if something is an Array.

This is what I want:

if (isJSON(data)){
    //do some data stuff
}else{
    //report the error
    alert(data);
}