how can tell if a column is a primary key?

Hello! 
I'm writing some perl code to access arbitrary Postgres tables. As part
of this, I would like to be to tell if a given column is a primary key.
Given a table and a column name, is there is a select statement I can
run on the systems tables to tell me whether or not it's a primary key? Thanks.
  -mark

-- <<-------------------------------------------------------------->>
personal website                <    Summersault Website Design  http://mark.stosberg.com/     >       http://www.summersault.com/