How to display sql query fired by posts_where

The quick and dirty way is to put var_dump($wp_query->request) (or whatever you’ve named the WP_Query object if you’ve created a new one) on the page the query runs on and after it runs.

You can also add define('SAVEQUERIES', true); to wp-config.php to save the queries for analysis. You can read the queries something like this from the Codex:

if (current_user_can('administrator')){
    global $wpdb;
    echo "<pre>";
    print_r($wpdb->queries);
    echo "</pre>";
}