/**
 * Checks if a particular user has a role. 
 * Returns true if a match was found.
 *
 * @param string $role Role name.
 * @param int $user_id (Optional) The ID of a user. Defaults to the current user.
 * @return bool
 */
function appthemes_check_user_role( $role, $user_id = null ) {

    if ( is_numeric( $user_id ) )
	$user = get_userdata( $user_id );
    else
        $user = wp_get_current_user();

    if ( empty( $user ) )
	return false;

    return in_array( $role, (array) $user->roles );
}

// example use for the current user
if ( appthemes_check_user_role( 'customer' )
    _e( "You've got access dude!", 'appthemes' );
else
    _e( "Sorry man, no luck.", 'appthemes' );

// example use for a specific user
$user_id = 23;

if ( appthemes_check_user_role( 'customer', $user_id )
    _e( "You've got access dude!", 'appthemes' );
else
    _e( "Sorry man, no luck.", 'appthemes' );