When someone says: How right he was, it means that I agree with what someone said.
EG: He said Mike should be fired as soon as possible. How right he was.
Yes. The expression also has the sense that the person saw into the future better than others around him/her:
Everyone scoffed when he told us not to give Bernie Madoff any money to invest, but how right he was!
Thanks to you it's much clearer now!