How to know what functions are hooked to an action/filter?

Look into the global variable $wp_filter. See my plugin for a list of all comment filters for an example:

<?php
/*
Plugin Name: List Comment Filters
Description: List all comment filters on wp_footer
Version:     1.1
Author:      Fuxia Scholz
License:     GPL v2
*/

add_action( 'wp_footer', 'list_comment_filters' );

function list_comment_filters()
{
    global $wp_filter;

    $comment_filters = array ();
    $h1  = '<h1>Current Comment Filters</h1>';
    $out="";
    $toc="<ul>";

    foreach ( $wp_filter as $key => $val )
    {
        if ( FALSE !== strpos( $key, 'comment' ) )
        {
            $comment_filters[$key][] = var_export( $val, TRUE );
        }
    }

    foreach ( $comment_filters as $name => $arr_vals )
    {
        $out .= "<h2 id=$name>$name</h2><pre>" . implode( "\n\n", $arr_vals ) . '</pre>';
        $toc .= "<li><a href="#$name">$name</a></li>";
    }

    print "$h1$toc</ul>$out";
}

Sample output for pre_comment_author_email:

array (
  10 => 
  array (
    'trim' => 
    array (
      'function' => 'trim',
      'accepted_args' => 1,
    ),
    'sanitize_email' => 
    array (
      'function' => 'sanitize_email',
      'accepted_args' => 1,
    ),
    'wp_filter_kses' => 
    array (
      'function' => 'wp_filter_kses',
      'accepted_args' => 1,
    ),
  ),
)

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