I have did it also many times before. It is fun to do and yes an ultrasound is your only way to find out what you having.
I've tried it and it did a circle every time lol I looked it up on YouTube too. It's a fun little experience
It has a 50/50 change. The only way you will KNOW 100 % is by a ultrasound. They are tho fun tp do. So dont take.them to serious when done :)