“你怎么知道”英语怎么说？

how do you know：你怎么知道？How do you know that I arrived?你怎么知道我来了呢？
How do you know I have a pretty
girlfriend?
你怎么知道，我有一个漂亮的女友？

How do you know anybody gave me anything?
你怎么知道有人给我了什么东西呢？
Now it"s my turn to ask you. How do you know that I have a little
secret?
现在该轮到我提问了：我有一个小秘密，您是怎么知道的？