define.php

<?php
/**
 * Check if content is JSON
 *
 * @param string $content
 * @return boolean
 */
function w3_is_json($content) {
    return json_decode($content) !== NULL;
}

/**
 * Check if content is JSONP
 *
 * @param string $content
 * @return boolean
 */
function w3_is_jsonp($content) {
    // Extract json
    $stripped = preg_replace("/^\s*\w+\s*\(/", "", $content);
    $stripped = preg_replace("/\)\s*$/", "", $stripped);
    return w3_is_json($stripped);
}

http://wordpress.org/support/topic/plugin-json-api-w3-total-cache-json-api

http://wordpress.stackexchange.com/questions/50324/how-to-cache-json-with-wp-super-cache