You don’t know that. One of the key reasons that COVID-19 has proved such a destructive enemy is because a significant percentage of transmission happens when people are asymptomatic . According to the latest reports from the Centers for Disease Control in the U.S., more than a third of people with the virus never show symptoms . Which makes the whole “I’m sick/I’m not sick” conversation kind of moot. There are some exceptions to the current mask guidelines (including children under two, people who have trouble breathing, anyone who, for any reason, might have trouble removing a mask on their own, for example), but, for the most part, every person who has not been in bubble wrap for two weeks needs to conduct themselves as if they have the virus, and if that’s the case then — I think you see where I’m going here — just put a mask on already!