That’s the question that even published authors have difficulty answering. If you have fans writing wikis and fanfiction based on your stories, then you’ll know you’ve got an audience/fan base (or at least had one before going on homophobic/transphobic rants on twitter). Movie deals help
Per chapter & month does help, a bit. Simply cause you know they’re at least hitting “next-chapter” button. My main potter fanfic on FFN, for instance, has 420 views & 39 visitors so far this month. Of that, I see 48 views/29 visitors to the first chapter, while subsequent chapters are 3-8 views/3-5 visitors. The day-day histogram shows that about six bots came through, drowning out the metrics (or humans clicking next-next-next w/o reading or doing very-fast-skimming). I do know, though, many of the “hits” come from people finding a link to the story, maybe from another author’s favorites, and deciding to move on (or bookmarking it for later).
AFF & AO3 only have the whole-story hit counters. By recording these daily, I know that a recent post can generate one to two hundred hits/day, but it’ll settle down to the single-digits per day within a week or two. However, as they’re only whole-story, I don’t know if it’s something browsing the first chapter & moving on, vs reading the whole thing & enjoying it. AO3 has Kudos and number of bookmarks/subscriptions available, so that helps measure some level of interest. FFN has the favorites & alerts, also relevant.
But, when it comes to knowing that readers enjoy the story, reviews are that golden ticket, the best measure. However, maybe one reader in a hundred will leave a review, if that. (My *HIGHEST* review count here on AFF is NINE, the average is around 2-3 per story. My biggest is that main potter fanfic on FFN, with 180.)
Moral of the story, if you want to trend set, try to always review when you read.