triggers: how to check if a field changed?

hi list

for our discussion board i've successfully set up a trigger to have tsearch2 index the record every time it is updated. this works fine, but also triggers the reindex of the record every time i update the "# of views" counter, which creates unnecessary load. in a trigger, is there a way to check the fields affected by the UPDATE query? i only want to have the tsearch2 index updated when the indexed field (p_msg_raw) changed...

this is the current trigger (straight following the tsearch2 guide):

CREATE TRIGGER "posts_ts_update" BEFORE INSERT OR UPDATE
ON "forum"."posts" FOR EACH ROW
EXECUTE PROCEDURE "public"."tsearch2"(idxfti, p_msg_raw);

thanks in advance,
thomas