We usually want to know whether someone is a good person or not when we meet them. This cannot be determined by external examination only though.
People often conceal their true nature, projecting a false image.
Thus when you have known someone only a short while, you don't really know him. This doesn't mean today's method of traveling. This is a very old proverb, from the days when one traveled on foot or on horseback and stayed in the same room with others. When traveling together under such circumstances, you quickly get to know your traveling companions, discovering what kind of character they have and how high their ideals are. So when we want to understand someone we not only examine his external appearance, but his way of life as well.