Set up a fake menu item to test using an icon font.

admin-icon.php

<?php
/**
 * Plugin Name: Admin Icon Example
 */

// Add the fake menu item
add_action( 'admin_menu', 'admin_fake_menu_item' );
function admin_fake_menu_item() {
	add_menu_page( 'Icon Test', 'Icon Test', 'edit_posts', 'admin-icon-font', '__return_true' );
}

add_action( 'admin_head', 'set_custom_font_icon' );
function set_custom_font_icon() {
?>
        <style type="text/css">
                /* for top level menu pages replace `{menu-slug}` with the slug name passed to `add_menu_page()` */
                #toplevel_page_admin-icon-font .wp-menu-image:before {
                        font-family: FontAwesome !important;
                        content: '\f00d' !important;
                }
        </style>
<?php
}

// enqueue custom icon font
add_action( 'admin_enqueue_scripts', 'enqueue_font_awesome' );
function enqueue_font_awesome() {
        wp_enqueue_style( 'font-awesome', '//netdna.bootstrapcdn.com/font-awesome/3.2.1/css/font-awesome.css', false, null );
}

thank you a lot.
this code is really practical.