Do a DNS lookup on your salesforce hostname and it will be an alias to a hostname that contains the datacenter in it. Note that these can change during failover operations and maybe for other reasons.
For example, NA2 is currently hosted in CHI:
$ nslookup na2.salesforce.com
Note the na2-chi. That would be the CHI airport code - so somewhere in Chicago.
Again for cs6:
cs6.salesforce.com is an alias for cs6-tyo.salesforce.com.
cs6-tyo.salesforce.com has address 184.108.40.206
cs6-tyo.salesforce.com has address 220.127.116.11
So TYO - Tokyo.
Interestingly enough, the location based on the IP address alone is Singapore.
*This is not a documented behavior and may be subject to change. If you need an authoritative answer you should contact your account exec/support.