You may know by a handful the whole sack

The quality of the contents of a sack of corn, flour, etc., may be judged by the quality of a sample. Putting it another way, we do not have to eat the whole egg to know that it is bad.

The proverb means that we can judge a person’s character on very small evidence. For example, a generous action suggests a kindly nature, and a callous remark a hard, unfeeling heart.