You may as well ask "how do you know if you have mail" e.g. physical mail. I see it when I go to leave the house. Maybe I hear the postbox ring. I don't need

You may as well ask "how do you know if you have mail" e.g. physical mail. I see it when I go to leave the house. Maybe I hear the postbox ring. I don't need an immediate notification of e-mail, it's quite fine to batch it and see if anything interesting is there. Daily, or a few times a day, or maybe after a week, depending on what I'm doing.