Without AF, how do you KNOW you're not pregnant?

vszapp

DD is 3.5 months (EBF) and still no AF. Since we are planning to get pregnant again after DD is 12-18 months, I have decided not to get an IUD. Also, I prefer not to use the minipill because I get migraines. So, DH and I have been using condoms - *sigh*. Not that we really get a chance to DTD much lately anyway, but I still worry that I won't know if I actually get pregnant accidentally until very late in the game. Just curious if there are any typical signs while you are BFing that I could watch out for just in case. But of course, prevention is key!