Te conozco como si te hubiese parido

[ROBINDESBOIS]

When we know sb very well, especially the way he thinks or if he hides sth. English?

[Rusty]

To know someone like the back of one's hand. ('to know something like ...' is more common, but it's used)
To know someone as well as one knows oneself.
To know someone as well as one knows one's child.

I know him like the back of my hand.
I know him as well as I know myself.
I know him as well as I know my son.
He could be my son, I know him so well. (I know him so well, he could be my son.)

[JPablo]

Also,
I can read you like a book.

Related expressions, "conocer el paño" [to know what’s what/ to know what/who one’s dealing with] "conocer el percal" [to know the score, to know the people (o situation, etc) one is dealing with] "saber por donde andan los tiros" [to have an idea of something (or of what’s happening)]