How can I tell someone public address?

Please clarify your question.

I need to find out the public address on a computer, the owner does not know it

Hmmm, unless the computer in question is directly connected to the internet, the closest you'll come is their access device, normally firewall, router or modem.

There are a number of sites that can tell you that information, for example: Internet Broadband Speed Test, IP Address Lookup, DNS Lookup, VoIP (http://www.internetfrog.com/)

Thank you so much for your help!

At a command prompt type IPCONFIG /ALL