The Talmud in Megillah 14a states that Yiskah is Sarah:
ואמר ר' יצחק יסכה זו שרה ולמה נקרא שמה יסכה שסכתה ברוח הקדש שנאמר כל
אשר תאמר אליך שרה שמע בקולה ד"א יסכה שהכל סוכין ביופיה
and R. Isaac said [on this]. Yiscah is Sarah; and why was she called
Yiscah? Because she discerned [sakethah] by means of the holy spirit,
as it is said, In all that Sarah saith unto thee, hearken to her
voice. Another explanation is: because all gazed [sakin] at her
beauty. (Soncino translation)
This identification is also made explicitly by Targum Pseudo-Jonathan on the verse in question, adding an additional two words at the end of the verse saying "she is Sarai":
ונסיב אברם ונחור להון נשין שום אתת אברם שרי ושום אתת נחור מלכה ברת הרן
אבוי דמלכא ואבוי דיסכה היא שרי
Many of the standard biblical commentaries adopt this identification as well, some suggesting that if Yiskah was not Sarah there would be no point of introducing her in this verse.