Board Thread:General Discussion/@comment-24518534-20140207202148/@comment-26420452-20140208201935

They had to know, they were monitoring all the battles but I think they already knew.

Mayuri of course knows about it. In chapter 516 he says that for research purposes, he has knowledge of every soldier's Bankai. And he mind-absently told Ichigo about Ikkaku's Bankai.

Judging by Iba's conversation with Ikkaku, and Kensei's with Hisagi, apparently you can tell if someone has Bankai (I guess your Reiatsu composure changes when you obtain Bankai and the ones familiar with it will notice).