How to translate "For all you know"?
Agree with StumpyJoePete. 

As we all know = Everybody knows.

For all you know = You don't really know for sure. 

I think Damian Siniakowicz's suggestion will work. 

We better wash the vegetables first, for all we know, they could be covered in pesticide

我们还是先把蔬菜洗一下，说不定有農藥呢。

A couple more suggestions:

我们还是先把蔬菜洗一下，誰知道有沒有農藥呢？

我们还是先把蔬菜洗一下，可能有農藥呢。