She gets emotional during her periods and develops cravings for weird foods when she starts ovulation. She's very regular which makes it that much easier to keep track. I know her body as well if not better then she does... and that's not false modesty. She says the same.
There was a night when I wanted to get intimate and go all the way with my girl. After some making out and stuff, she realized what I had on my mind and told me that her period is usually on "These" days. I just remembered it, and that's it.
Guys don't know (unless we tell them, they see it, or they count days) but they do like to guess whenever we are a bit irritable. Good rule of thumb gentlemen: Never tell a girl when she is on her period.
If my boyfriend or husband wanted to count days, that wouldn't bother me. He can be better prepared for my ice cream binges. ;)
The guy I was with for years always knew. But since we both didn't want a kid it made us both happy to see a little reassurance on top of my bc. Plus he liked to get me chocolate as a preemptive messure so I would rrlease my wrath on him.