From what Yitzchok says in Bereishis 27:22 “the voice is the voice of Ya’akov, and the hands are the hands of Eisav” it seems that he had a doubt as to the identity of the person who had come to receive his blessing. If so, why did he go ahead and bless him, instead of waiting to make sure?
The sefer Tsror HaChaim (added to beginning of the sefer Shoshanas Ya’akov) here explains:
Yitzchok was afraid that Ya’akov might come and trick him, as he in fact did. So what did he do to forestall this possibility? He figured that if Ya’akov was going to try to trick him into thinking that he is Eisav, he would change his voice and speak harshly like the voice of Eisav. Therefore he instructed Eisav that when he returns from hunting to receive the blessing he should change his voice and speak gently like the voice of Ya’akov. Thus, if the one coming for the blessing spoke like Eisav, he would know for sure that it is Ya’akov.
But Rivkah knew about Yitzchok’s plan through prophetic vision, as we see from an earlier posuk “And Rivkah spoke to Ya’akov, saying (לאמר): Behold, I heard your father speak to Eisav your brother, saying” (27:6). The Alshich asks why the posuk writes the word “saying” twice, but we can explain it according to the well known rule that wherever the Torah writes an expression of דיבור it connotes a harsh way of speaking, and wherever it writes an expression of אמירה it connotes a gentle way of speaking.
Hence, the posuk means that “Rivkah said to her son Ya’akov, לאמר” - she told him to speak gently and not to change his voice from how he normally speaks. And why did she tell him to do this? Because “behold, I heard your father speaking to your brother Eisav, לאמר” - he told Eisav that he should speak gently. Therefore if you change your voice and speak harshly like Eisav, he will realize that you are Ya’akov.
Thus, when Ya’akov came before Yitzchok he did not change his voice, and therefore Yitzchok confidently said “the voice is the voice of Ya’akov” - that is one sign that this is Eisav, “and the hands are the hands of Eisav” - that is another sign. And so “he blessed him” - because he had no doubt in the matter.
The full text in English can be found here.