I suppose this fits under Health. Does anyone know whether or not a doctor can determine if you are still contagious from a virus?

A colleague of my husband’s had hand foot and mouth disease, full symptoms (fever, rashes) earlier this week from Sunday- Thursday, and now says he feels fine and claims the doctor confirmed he is not contagious just 3 days later, so he is back around people (next to my husband) working. Anyone have any familiarity with doctors doing specific testing to see if you are contagious after a virus?