一叶知秋----The falling of one leaf heralds the autumn

例句:
A: It’s said that he can forecast things in future.
听说他能够预知未来。
B:That’s because he knows well that the falling of one leaf heralds the autumn.
那是因为他善于一叶知秋。

同义词:
It is a straw in the wind. A small sign can indicate a great trend.