How do I know when i’m fertile..?

I am on the pill and me and my partner don’t usually wear condoms when we have sex; and I’ve read that when you’re fertile or ovulating you have more of a chance of getting pregnant which is what i’m trying to avoid. So i’m thinking, since I want to be extra safe, (as I sometimes don’t take the pill so strictly as I should) I would hold off from sex or be extra cautious when I AM fertile. I just don’t exactly know how to tell when I am or not 😬 help plz