how does he know!??? as if Tea-min is the only one who has badass personality. (￣∇￣"...

I think Si-woon tested him to "confirm his reaction" via rubbing his lips and sticking his finger in his mouth. You're right, Tae-min isn't the only one with a badass personality, but anyone with that personality that WASN'T Tae-min likely wouldn't have let Si-woon just casually stick his finger into his mouth without biting him. ;)

I think before Si-woon did that to si-won (rubbing his lips and sticking his finger in his mouth), he already had "Si-won is Tea-min" in his mind. And Si-won's reaction that Si-woon wanted to confirm here was that what he would do after he watched the CD.
Sorry for my bad Eng ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄

Yes I think that, too; I just like how this sequence added another layer to Si-woon's assumption.