So, when you know which way the wind blows, you know what is going on.

所以，当你看清形势（which way the wind blows）的时候，你知道会发生什么。

But the meaning is clear when he sings " you don't need a weatherman to know which way the wind blows."

但是这次意思很明显，他唱到“你并不需要气象员来告诉你风往哪边吹”。