MySQL trigger: Compare value in another table
DROP TRIGGER IF EXISTS post_queue_insert;
CREATE TRIGGER post_queue_insert AFTER INSERT ON posts
FOR EACH ROW BEGIN
    DECLARE found_it INT;
    IF (NEW.post_type != 'revision' AND NEW.post_status != 'auto-draft') THEN
        SELECT COUNT(1) INTO found_it FROM some_other_table
        WHERE post_type = NEW.post_type;
        IF found_it = 0 THEN
            INSERT INTO event_queue (action_id, action_do, action_key, action_value, action_event, action_timestamp, sync_complete, SITE_ID) VALUES (NEW.ID, 'post', NEW.post_type, NEW.post_status, 'insert', UNIX_TIMESTAMP(now()), 0, 1);
        END IF;
    END IF;
END;


If found_it = 0, then the row does not contain a value matching NEW.post_type.

Give it a Try !!!