We've all experienced this awkward moment: You've known someone for months or even years and are suddenly required to know his last name, only to realize you forgot or never knew it. When it's inappropriate to ask, there are ways to find a person's last name on the sly.
Most offices have a website listing their primary staff, if not all their employees. Check the contact list for the office where the person works, and search by her first name.
Ask the person to send you an email. Most people, when setting up their email addresses, include their first and last names.
If you work with the person or are in the person's house and see mail lying out, glance at the address label when the person is not looking.
Ask the person when and where he graduated from high school. If the high school is nearby, look up the yearbook from that year and scan through the pictures. The person's first and last name will be listed.
When all else fails, ask the person how to spell her last name. This will make it appear that you know it already but have simply forgotten how to write it, and it's a gracious way to get the information.