Confused about Facebook?

[pri✍]

Agree with the two previous answers, just want to add something, they might lie to you.

You can check whether they have seen it or not by seeing the last message you sent to them, if they read it, most probably you'll see on the bottom of that last message "Seen at...". I said most probably, because sometime FB don't correctly show that or after they read it, they click 'Mark Unread'.