You definitely want to take the folic acid because it helps prevent some very early pregnancy birth defects.
You had sex about two weeks after af, but how long are your cycles normally? You ovulate about two weeks before af and you are fertile for about three days and on the day of ovulation. If you have 28 day cycles, and you had sex every other day, you probably had sex on your fetile days.
Some women who ttc (including me) try to pinpoint ovulation better either because of irregular cycles or to verify when ovulation occurs. You can pinpoint ovulation several ways: looking at changes in cervical mucus (cm), tracking the position of your cervix, taking ovulation predictor tests (opk), using ovulation monitors, and taking your temperature every morning before getting out of bed (BBT). BBT (also called charting) will indicate that you ovulated on a certain day, but will not tell you when you are about to ovulate. The other indicators are the opposite - they will tell you when you are about to ovulate but won't indicate actual ovulation.
One word of caution before you start trying to predict ovulation - doing so can cause you to think about ttc a lot. It can lead to you getting really sad if you get af or a bfn. Make sure you have other things in your life to help you feel like you are living as opposed to feelling like you can't live until you get pregnant.
