Whenever I am reading text books or articles of importance for learning I tend to decide what is important by first skimming the text for bold or italicized words followed by their definition or their explanation of importance to either the reading or to general knowledge. After skimming the text I then start my reading and pay attention for key parts that seem like they might be important. I usually decide what is important either by noticing that it pertains to something learned in class or if it is something that I thought was interesting or if I learned from it. If I learned something from what I just read then I consider it important and usually mark it by highlighting it or underlining it with pen. Same goes for if I think something is interesting. If I find something interesting or new to me then chances are I might need to know it for later purposes.
When I read I normally have an internal conversation with myself about the text. I can hear my internal voice reading the text along with me whenever I am really paying attention to what I am reading. This is how I know if I am still on task with my reading or paying attention. If i do not hear my internal voice I know that I really was not paying attention let alone comprehending what I just read or presently reading. When I do not hear my internal voice I get myself back on track by rereading what I just did or by taking a short break to think about whatever was distracting me and then start my reading back up again.
I know that something is important in text whenever it is bold or italicized like I mentioned before, but I can also tell something is important in the text if there are reflection questions in the back on certain parts of the reading. Usually textbooks or articles are organized by major subjects that it is addressing and then under each major subject are little subtopics. I can usually count on those little subtopics having meaningful information in them. The subtopics are the one that are getting down to the point usually and all the other stuff is just recapping on those subtopics are building up them.