How do I list the groups that a UNIX user is a member of?

With pts m groupname I can list the users in a group. How can I do the opposite - that is, list all the groups that a user is a member of?

(I need this to find the name of a group that I know a user who is a member of, but I don't know the group name...)

Clarification:
This question is complicated by the fact that I am not the system administrator, and that I don't have root permissions (far from it). Instead, I am part of a group of "moderators" that administrate a small part of the system - to be specific, the physics branch of the student union at my university's central IT system.

Within our branch, we have a bunch of different access groups for people who are in charge of things. In this case, one of two people responsible for something (it doesn't matter what) have been replaced, and I was looking to examine the other user to find out what access rights I should give the replacement.

I have now been able to solve the immediate problem (the new guy not being able to access a folder) by examining the folder to see who has access, and there picking the group from the list. However, there might be more privileges that this user should have that I don't know of, so the question is still relevant for me, albeit not so acute.

You can also use the groups command:

[root@ftp ~]# groups root
root : root bin daemon sys adm disk wheel

if all else fails there is also good old grep:

[root@ftp ~]# grep root /etc/group
root:x:0:root
bin:x:1:root,bin,daemon
daemon:x:2:root,bin,daemon
sys:x:3:root,bin,adm
adm:x:4:root,adm,daemon
disk:x:6:root
wheel:x:10:root,admin

$ id [username]

Found it - way later, but I did! =)

Just as

$>pts m [group name]

lists all the members in a group,

$>pts m [user name]

lists all the groups a user is member of. It was too simple :P