I think its more likely that Cadsuane knows balefire than unlikely. She's never shown a deficiency in any knowledge compared to another Aes Sedai, and her ability to invert makes her unique and means she's been exposed to hidden knowledge. No AS seems to know more than Cadsuane about saidar, and therefore if Moiraine knows balefire, then I find it likely that Cadsuane took some trouble to find out about it at some stage. The story implies that she's the oracle of the Aes Sedai. Nynaeve is very happy that Cadsuane doesn't know the function of some of her ter'angreal which implies that the woman has an irritating knack about knowing everything (in the Tower of course... the Aiel, Forsaken etc know things she doesn't).

However, if you want to go on proof alone, there's no proof either way. My experiences of Cadsuane make me bet that she knows its weave.