Dude, it is easy to know whether they are listening or not. You just ask them to repeat your question or topic or the last sentence you just said. If they can do that, it means that they are listening attentively. If they can't, I am sorry to tell you that you may need to change a listener or change the topic because maybe they are not interested in your topic. Good luck!
It is said that our blink rate is less than half when we are attentively listening. Maybe it is true. Every time when I pay attention to reading or someone's talking, I often stare at him or her and never look at other place because I am afraid that I will miss some important info. And there is research that can prove this is true. Researchers at the University of Waterloo have found that people blink significantly more often when their minds wander than when they are focused. I used to doubt that why my teacher would knock my desk every time I was absentminded in class. Maybe this is the reason why teachers always knew I was single out in class!