What moment in the books made you realize that Snape was the Half-Blood Prince when reading book 6 for the first time, without having any doubts that it was somebody else?

For me, it was when Snape was holding the 'Roonil Wazlib' textbook and said "you are a liar and a cheat" in the boys bathroom after the Draco vs Harry showdown. That's when it all clicked for me.