Why did Yitzchak question Esav's identity?
He questioned him because he returned from the hunt so terribly quickly, as the Torah specifically writes (in Pasuk 20).
He was probably also surprised by the first word uttered by Ya'akov "Avi", 1 said to him directly, whereas Esav tended to speak indirectly, as we see later, when (in Pasuk 31, he began with the words "Yakum Avi ..." (in the third person). 2
Rashbam: Refer to 27:22:4:1.