Why did Yitzchak question Esav's identity?
He questioned him because he returned from the hunt so quickly as the Torah specifically writes (27:20).
He was probably also surprised by the first word uttered by Yaakov, "Avi," 1 said to him directly, whereas Esav tended to speak indirectly, as we see later, when (in 27:31, he began with the words "Yakum Avi..." (in the third person). 2
Rashbam: Refer to 27:22:2:1.
Just like he himself did at the Akeidah (See Bereishis 22:7. Interestingly, Avraham there responded with the same "Hineni" as he responded here to Yaakov).
Why did Yaakov initially say only "Avi "?
Malbim: He was afraid to say more. When he saw that his father did not recognize his voice, he continued "Anochi Esav Bechorcha."
Ha'amek Davar: He waited to see from his father's answer if it is a proper time for Yitzchak to prepare himself for Ru'ach ha'Kodesh and to bless. Yitzchak answered affirmatively - "Hineni."
