“知根知底”用英文可以这么说

“对某人或者某事很熟悉”可以使用：
Be familiar with
eg: I'm familiar with him.
或者，
Know someone very well
eg: I know him very well.

如果熟悉程度达到连小秘密都知道，应该怎么表达呢？

Know someone inside out

含义：

"Know someone/ something inside out" is:

If you know something or someone inside out, you know them extremely well.

知根知底，了如指掌。

例如：

I just think you should know a person inside out before you decide to marry this person.

我觉得在你决定嫁给一个人之前，你应该对这个人知根知底了如指掌。

If you study this material for weeks, you'll know it inside out by the test date.

如果你花上几个星期研究这些材料，到考试日期你就会完全掌握了。