Look at the "Attribute Editor" tab against a user account in "Active Directory users and computers." It should be in the list there (uidNumber) amongst other useful things.
I'm no Microsoft administrator but am aware that this "Attribute Editor" tab is not listed if you search for the user; rather you have to browse to the user through the tree.
Unfortunately, I do only have LDAP access to our AD. AD is managed by a separate department.