Firstly, google's DNS service (which is provided on the IP addresses you list above) is anycast, which means there are systems all over the world that provide service on those addresses, and you get directed to the nearest one to you when you request it; their main website is similarly distributed. Try pinging 8.8.8.8 from the UK, and look at the RTTs - they'll likely be sub-30ms, so there's no way they're going transatlantic. Do by all means ping them as a general connectivity check, but don't assume they're in the US.
So how to find an address to ping to check routing to a DC? In my opinion, you should talk to the DC. They know what parts of their infrastructure they use to participate in BGP routing; they will know which parts of their address space they will reliably present to the outside world by whichever the best-connected path is at any moment. You have a contractual relationship with them, as you're hosting equipment with them, so they have some incentive to answer the question. And if they're worried about someone pinging their core router once an hour, you know that they're sufficiently off-the-wall that you should find yourself a new DC.
