I usually know because I dream of babies. I once had a dream I was chasing little naked babies through the veggie garden. But I knew this time for other reasons. For one, DH had the baby dream this time (weird). I also felt the egg implant, which should be impossible but I darn well felt it. I also ovulated early last month, which caught us (natural BC method) offguard. And lastly, yet the strongest reason is... I just knew. I can't explain it, I just knew. I even took a pregnancy test a week
before AF was due

, and it was negative (hey, the test said 99% accuracy 5 to 7 days after implantation!) I still felt sure, so the day AF was due, I took another test - positive. I have months where I
think I know I'm pregnant but it isn't the serene surety I have when I actually am. I've discovered there's a difference in how it feels for me.
That same surety hit again when I felt like I was losing the baby. My temp dropped and my breasts didn't hurt anymore. I don't have a doc so I just went to any ol' one and he sent me for an US which showed a smalled hemorrhage. A few days after that, all signs came back and I'm as sick as a dog - which is a good sign.
Oh, I forgot the other physical sign I had. I feel the cold, I love the heat. It's winter here and for a couple of weeks I kept saying, "Hey, spring is on the way" and took a layer off the bed. Only to realise the temperature hadn't changed, I had.