Depending on where you get them from, there's a very thin line that differentiates the fake followers from the real ones.
Instagram has been super strict and banning accounts left, right and centre so the trashy ones have already faded away.
And with time, several sellers have gotten smarter, their accounts post often, the ratios are maintained and they even have realistic usernames and bios.
On the other hand, there are real users who don't use IG frequently, don't upload pictures/only upload memes, follow a lot of people but aren't popular enough so don't have enough followers.
Add all that up and heck you and me would find it difficult to identify what's real or not, so an algorithm? Nah, I don't think that will be able to do that.
This is the reason why the majority of the tools that even do that are flawed and none of them will be able to give the exact numbers.