User blog comment:Lifebleedin/how does killua know zenos ability/@comment-24287488-20140123063005/@comment-24287488-20140123070831

I know that you were going to mention it. Mizuken (His Nen teacher) did not fire a Nen bullet. He simply throw an acorn with great intensity at Kurapika. That's why he was not able to see it but was able to sense something coming towards him.

And lastly, if I am wrong then it is definitely Zeno himself said about his ability to Killua. If I remember correctly, in a certain episode Killua said that his granfather talked about his adventures with Killua. They are family members afterall. How can they not know each other abilities?