I feel that may be at this point Asad believes Tannu but after the murder attempt.. He will put two and two together.. think about other attempts on Tannu and how they stopped after the Epic SR.
He will use his grey matter and think about the events of the past .
The problem here is apart from Tannu and the audience ... the characters do not know what happened that night. Zoya knows it was not Asad's fault but she herself is not 100 % sure.
She will think that it might have been date rape and so maybe Tannu is pregnant.
Or maybe in over confidence Tannu will blurt the truth.
Because the way Asad is he will only believe it if it comes out of Tanu's mouth...