Relatively simple question about getting attributes from the where clause

Hi guys, I am new to postgres hacking, and I have the following question.
Say that someone executes a query:

SELECT * FROM user u, logins l WHERE u.id = l.id
(assume I have tables user, and logins, with id fields).

After this query has been parsed and created into a plan tree, how can I
get the char* form of the attribute name on which my WHERE clause searches
on (in this case, user.id and logins.id)?

Thanks a bunch guys
-TJ