Chinese Proverbs 察其友，知其人

chá qí yǒu, zhī qí rén
察 其 友， 知 其 人

A man is known by the company he keeps.

解释 ( paraphrase )
观察一个人有什么样的朋友，就知道这个人的性格和为人。
这条谚语告诉我们，人与人志趣相投而变成朋友，而且能相互影响。一个人要是有良好的品行，那么他的朋友也同样高尚。如果他品行低劣，那么他的朋友也不会太正直。因此从一个人的朋友就能看出这个人的性格和为人。这里建议我们要结交善友，善于结友。品行好的朋友能使人受益终身，品行差的朋友会让人享乐懒惰。

This proverb tells us people of the same tastes and habits like to be together and influence each other. If one behaves well, his friends are also noble-minded. If a person behaves badly, his friends are not supposed to be honorable. Therefore, a man can be known by the company he keeps.
The proverb advises us to make good friends and right decision. A well-behaved friend can benefit you and an immoral friend will lead you to enjoyment and laziness.

察：观察。
      To observe.

造句 ( example )
表弟好像不爱学习了，古人说“察其友，观其人”，看他的朋友们都喜欢通宵打游戏，估计他也好不到哪儿去。