How can I find out what someone's maiden name is?

If you know the State and County she lived in when she got married,
you can go to that county probate court and search the applications
for marriage license.  Depending where and when she was married, a lot
of this information is online.

It doesn't actually matter if he or she is a maiden or not
easily go to:

http://www.peoplefinders.com/

You just need one of the two options:

1) First name, middle name(if he/she has), last name, date of birth,
city and state.

OR

2) Phone number.

Sorry I made a mistake.
The options are the ones I last posted and one more:

3) Address.