It's not that hard to spot. A woman usually releases one egg a month from her ovaries into the fallopian tube. The release of a mature egg occurs about 14 days before the start of her menstrual cycle. The first day of a woman's period is considered the first day of her cycle. The egg has to be fertilized within 24 to 48 hours of release.
Fertilization takes place in the fallopian tubes.
Of the 500 million sperm that are ejaculated, about 250 make it to this point, but the fallopian tubes are a much more scenic and inviting territory for the sperm. They can hang around for up to a week just waiting for that egg. Most women's periods are not a consistent 28 days and you can not really be sure. Add to that, the increased risk of getting a sexually transmitted disease during a woman's period. It's best just to use a safe and reliable method of birth control correctly, every time you have sex.