Unless you're using a script to measure this, this is impossible to read, especially for boards that are 20 years old. There's no way to tell, because some members have even died in that time (I've only seen it on one forum where the member was there the entire time till their death).
Unless the real users mix in with the fake and the pendulum swings to where it's legitimate users. But then, I revert back to my first statement that real users will always come and go. There's literally no way to measure this accurately. You are just eyeballing it, and probably better at it being a forum admin than the average user.
Your initial user base generally makes up the core of your community, at least unless there is a falling out. When those users disappear there are usually questions, whether or not people were there at the time or if they come by later and run across old content. If you have 21 active users at the start of your community, and only 1-5 of those users are active a year or two later... There are generally questions.
Also, unless it is AI, or paid posts, it is fairly easy to tell when the same person is sockpuppeting by the way they write, or different mannerisms. Not everyone will pick up on it consciously, but over time people do pick up on it sub consciously because your brain likes to recognize patterns.
I also don't call out communities for fake activity; the examples I am thinking of are specifically when other people have brought it up, and normally there has been backlash for it. People do not like being fooled, and that is exactly what fake activity is.
Whether you agree or not, I don't really care as it's your community, and not mine. Whatever choices or mistakes people choose to make is theirs, I'm just posting so that people who are just getting into this hobby/business/idiotic mistake do not take bad advice

. If you have to use fake activity/AI posts/paid posts to gain activity in your community... You probably do not have much of a reason to be running a community at the end of the day, especially when even having a small friends group is usually enough to get started.