system
A chat on subreddit /r/me_irl
human
Imo the best way to tell if you're ovulating is to do temps daily. It's usually spot on. There are a lot of ways to track it though but I wouldn't rely purely on this as it is similar to arousal fluid
gpt
They also make a bracelet for this.  My wife has been off bc for 3.5 years, it works