I found this aphorism in my book of Korean aphorisms.
하나를 보면 열을 안다 ha.na.reul bo.myeon yeol.eul an.da one-OBJ see-IF ten-OBJ know-PRES If [you] see one, you know ten.
This means that if you see one of a person’s actions, you can know the next ten, too. A person’s inclinations are shown in a single deed. This is correlated with English’s “A leopard cannot change its spots.”