seeing is believing idiom saying
眼见为实;百闻不如一见
said to mean that if you see something yourself, you will believe it to exist or be true, despite the fact that it is extremely unusual or unexpected
I never thought Mark would get out of bed before lunchtime on a Saturday, but seeing is believing! 我从没想到马克星期六会在午饭前起床，真是眼见为实！