I know what you think about, beacase I have the same experience.
我知道我经历过 I know that I have experenced it,
知道你在想什么 and aslo I know what you are thinking (about).
Since i have experienced that i know what you think about.
i know what you think about,because i have the same feeling.
I know what I had experiensed, and I know what you are thinking about.