If you're using ExpressionEngine 2 and you want to have a global variable which you can use to conditionally include content in a dev environment just paste this switch statement into your index.php file at around about line 95.

EE2 dev conditional

switch ($_SERVER['HTTP_HOST']) {
	//development
	case 'dev.example.com':
		$assign_to_config['global_vars']['site_mode'] = 'dev';
		break;
	//default
	default:
		$assign_to_config['global_vars']['site_mode'] = 'live';
		break;
}