Can I Dynamically Detect Google?

Hi!

I'm the webmaster of multiple sites and would like to move some traffic to a newer website, mainly to test it but not an all-encompassing forward because I'm not trying to kill the old/current site and most importantly; I don't want Google being re-directed! I've come up with two possible plans and looking for feedback.

My guess is that this (PHP) will be the safest:

if($_SERVER['HTTP_USER_AGENT'] == 'google'){
	//Current Site
}
else{
	//1 in 10 gets forwarded to the new site.
	if(rand(1, 10) == 10){
		header('Location: http://www.newsite.com');
	}
}

Code (markup):

My other guess was finding a list of Google's data-centres' IP addresses and letting them view the current site whilst forwarding normal browsers but I haven't come upon any reliable or recent address lists.

In regards to the first idea, is it probable that Google might browse without the user agent? I'm presuming not as they purportedly acknowledge directives in robots.txt.

Thanks in advance for any input!

Edit: An example for anyone interested in testing this:

if(!substr_count(strtolower($_SERVER['HTTP_USER_AGENT']), 'google')){
	//(Probably) Not Google
}
else{
	//Google
}

Code (markup):