in most cases you cannot perform this action if they have privacy turned on so that only friends can see their personal info, if that is the case the options are, get to know one of their friends then look at it through their profile someday at their house etc, or if your clever you may be able to exploit facebook using iframes and log into multiple user accounts in one window. I would say they should have that exploit fixed by now but then again i don't test the security of sites like i used to when i was a little hacker.
other than that you have to go on facebook identify yourself, and then add them to your friends....