<?php
function has_filter($tag, $function_to_check = false) {
    $wp_filter = $GLOBALS['wp_filter'];

    if( false === $function_to_check ) {
        if( empty( $wp_filter[ $tag ] )) return false;

        // Make sure at least one priority has a filter callback
        foreach ( $wp_filter[ $tag ] as $callbacks ) {
            if ( ! empty( $callbacks ) ) {
                return true;
            }
        }

        return false;
    }

    if ( ! $idx = _wp_filter_build_unique_id( $tag, $function_to_check, false ) )
        return false;

    foreach ( (array) array_keys( $wp_filter[$tag]) as $priority ) {
        if ( isset( $wp_filter[$tag][$priority][$idx] ) )
            return $priority;
    }

    return false;
}