I’ve worked in a few big organisations and I’ve found that using Outlook to look up colleagues can be difficult if you don’t know someone’s first name. So I’ve often found it easier to lookup the full name of a user using the AD lookup command line tools.
Open up a command line (preferably with admin privileges) and enter:
net user /domain <username>
This should print out the username, full name and AD group memberships for the user, among other details.