Dynamic access to record column in plpgsql function

Question

How can I address a column from a record in a plpgsql function dynamically?

In the following snippet I have access to a variable entity.colname that contains the column that should be checked in the IF constrol structure. I'm looking into how I can replace the foobar part in the snippet below with entity.colname. Is that possible in plpgsql?

IF NEW.foobar IS NULL THEN
    RAISE EXCEPTION '% cannot be null', foobar;
END IF;

This is something that could but doesn't work.

IF NEW.entity.colname IS NULL THEN
    RAISE EXCEPTION '% cannot be null', entity.colname;
END IF;

The example above is just for illustration what I'm looking to achieve, don't judge the functionality. :)

I'm using PostgreSQL 9.1.