Though it is a common courtesy to say "Bless you!" when someone sneezes, people rarely do unless it is someone they know. The next time you hear or see someone sneeze, offer those kind words regardless of whether or not you know them. And if you have a tissue on hand, offer that, too!
You are acknowledging another person's misfortune and offering them well wishes. It may be that they are fighting a cold for the fifth day in a row, or maybe they have suffered a season of allergies. Or it could just be that one-time sneeze that we all have every once and a while. Either way a little compassion when someone's not feeling well goes a long way.
Whether you know the person or not, talk to them a little after they sneeze. Find out if they are sick. If they are, offer to help: bring them some soup or help them get safely home where they can rest.
Say "Bless you!" and make eye contact. Simply let them know you care with that one statement.
Find out why we say "Bless you!" when someone sneezes, and when it started.