I knew at my org's preference night. I was torn between 2 sororities also, but pref night just clinched it for me. They were both very sentimental but at mine I just "knew." Corny as this sounds it is like when you truly fall in love - you just know and it is very hard to put into words.
I DEFINITELY knew for sure when I was waiting for pledge pickup (you found out which org you were in when they knocked on your dorm-room door) and thought to myself "Oh $#%@, I hope it's not XYZ!" It wasn't.
