I knew I was having twins... Because I'm a huge believer that God answers prayer, and I had prayed for 10 years that we'd have a son, then identical twin girls with blonde hair and blue eyes.
Also, I have highly irregular periods, and ended up conceiving these babies on round 2 of Clomid. Plus, I was charting because of this, and the 3rd day post ovulation I knew it was twins because my temp rose THREE TIMES higher than it had in previous cycles, without the typical plateau they are supposed to go through until implantation. My spike was also twice as high as the pregnant charts other ladies had in books and internet groups.
Also, I was never sick with my first, and with this pregnancy, I was the sickest ever (haha): I actually felt sick enough at 9 days post ov. that I was hovering over the toilet (though never actually ended up puking). Then we got a quick positive result on a home test on day 16.
Oh, and at my first OB visit at 14 1/2 weeks, my fundal height was 23!!! When I went in for u/s 4 days later to check dates, we only saw one baby on the screen. I just patiently waited, knowing there would be a second... And sure enough! A fleeting view of a second spine streaked across behind the baby we were looking at... (or it might just have been the membrane between them).
Praise God! We're not peeking at the genders, but I'm putting in my bets for identical girls! =O)
I hope you end up finding your gut to be correct, too.
P.S. I've also had on my fridge for over a year two magazine clippings: One is a picture of infant twin girls, the other of grown-up twins -- both sets have blonde hair and blue eyes.