Hi,
I think I kind of knew from the start.
We found out at our 6 week scan and I said to the doctor then "Oh thank god, at least that explains why I am already showing!"
Although I do remember going to bed the night before our first scan, patting my stomach and thinking "Oh well, this will be the last night I can pretend there are two" How right I was - after that I KNEW there were two!
Best of luck with your pregnancy, however many you are having!