You can "read" something at many different levels. For example, you might be holding a book in front of your face while you're actually watching other people just for the sake of watching them. Or you might "read" an assignment because your teacher asked you to, but you just sort of look at the words and sentences and then close the book.
To really READ something, you should set out with the goal of understanding what the words and sentences mean. It's easy to make that your goal, but how do you know you've accomplished it? One way is to test yourself on the material you've read. Imagine what questions a teacher might ask you about the reading assignment. If you can come up with the questions (and the answers too!), you've been paying attention to what you've read. If not, time to read again.