By looking at them.
Once you get to know a someone well enough their name, or the name they are sharing, will show up above them. I am guessing NPCs will follow this. You have to talk to them enough to get to know them well enough to identify them by sight.
I do not think there will be big arrows pointing you to someone. You will have to remember who they were, what their name was, and where they are located. While keeping in mind they will move about as well.
Good news is that this isn't a quest based game. You do not have to return to someone to turn in a quest. You will have to remember who they are if you are selling them something you are making or made a contract to.
I think the Devs wanted to make players and NPCs look the same and even make it where you couldn't tell the difference. Makes someone disguising themselves as one harder to detect right off the back.