You want to time it, you track her eggs as they leave for fertilization. How do you do that you ask? Well here's the answer: MAGIC! Everyone woman is different so there is no set time that it is safest. Technically the safest you can do that is during, at the very end, like the last couple of days, of her period. Then you get blood on your stick.
Other than that there is always the pulling out, but if you haven't done it yet, then I wouldn't rely on that.
So in conclusion: JUST GET A CONDOM! They're not the expensive and they give them out for free at abortion clinics.