When you want to see if you look good, you look into a mirror. It reveals the spots and shows if your hair is untidy, so you can change it.
To know what a human is really like, look at their heart. The outward actions and appearance can be deceptive, but the heart shows the real issues that need to be dealt with.
Take time to understand your own heart and see what needs to change, but also make the effort to find out who people really are and take a glimpse into their hearts, rather than just trusting the outward appearance.