When you are at full draw you are looking into the NP through your pheripheral vision.You will notice a big circle and a small black dot.If your at your correct anchor the black dot will line up dead center in the yellow circle.If you are anchored wrong,incorrect grip etc the dot will not line up.
I have mine set up below so I can take a quick look to see if Im dead on,getting to the point now where I dont even look at it anymore,but it is a good reference point if in dought.