Not saying it’s the greatest idea ever, simply what I do because there’s no other metric here, aside from reviews. (And don’t think asking the reader a comprehension worksheet would work either.)
Now, FF.net has good enough statistics for me to ascertain that bots are coming through. I mean, I doubt humans are reading my 700k potter fanfic in a day. It was meaningful the couple of times I watched a slow progression through the chapters, that did let me know that somebody was willing to hit “next chapter”. On FF.net, though, they have other metrics, like alerts & favorite counts, to help know that readers are liking it.