Hey Smithsonian it will be different for everyone, but these are some things I have taken notice of.
My dick reaches a certain quality of plumpness ( feels and hangs heavy ), veins become more noticeable ( few notes about this; as I jelq I can see the blood filling the smaller veins more and more throughout the dick, it looks like lightening is striking in the sky; The closer I get to finishing my routine the veins take longer and longer to fade away ), the entire penis becomes a particular color of reddish/pink, and there is a sensitivity change.
Interested to know from others!