I was pretty certain, even though this is our first! My first symptom was tons and tons of CM, starting just a few dpo. That was new. Then I got way tired, had dizzy spells, a bit of queasiness, and just odd feelings in my abdomen. The real kicker was when my cervix moved--for a couple of days it actually pointed the opposite direction from usual. I asked around, and the people who had experienced that said it also coincided with their BFPs. I had sore boobs, too, but that's a normal PMS symptom for me. It didn't seem worse or better, but they started getting bigger pretty much immediately. OH was "impressed." I think I've already gone up a cup size.
Good luck to all!!!!