if you've got a pretty good nose, start paying more attention to your own smells, (when you go to the bathroom,etc) you'll find that you too can identify when you're close to your period, close to ovulating, etc. our bodies go through significant change every month, hormonal, biological, etc. these changes can and do manifest themselves in our odor.
personally, i've noticed even my underarm odor changes when i'm close to my period.