I am going to address an aspect of your question that most other respondents seem to have overlooked. It sounds to me that you're asking, how do I know that I've understood a particular concept well enough to be able to turn the page and keep reading?
I know some students who get bogged down when reading mathematics because they feel they have to understand each line thoroughly before they can proceed to the next line. While this style works for some people, for most people, this is usually not the best way to absorb a piece of new mathematics.
When I am trying to learn a new piece of mathematics, I usually start by latching onto an important theorem and making it a goal to understand that theorem. The theorem might not be stated right at the beginning, but I will turn ahead to see what the theorem says. If the theorem uses terminology that I am not familiar with, then I will go back and look for the definitions of those terms. Sometimes, to understand the definitions, I have to understand some preliminary lemmas, so I will turn to those lemmas and iterate the process. So instead of reading forwards, I often read backwards. On this pass, I will also usually skip the proofs. Only when I have a good sense of the overall structure of the paper or the book chapter, and know where I'm heading, will I then start reading line-by-line.
If you adopt this method of reading, then it will usually be clear when you're ready to move on to the next step, because you'll be reading with a purpose.
