What did Yaakov see on the face of Lavan?
Seforno: He saw that he had accepted the Lashon ha'Ra that Lavan's sons had spoken about him 1 (in the previous Pasuk, 31:1).
Since Hashem will command Yaakov to return (31:3), why must the Torah teach that he saw the hatred of Lavan and his sons?
Ohr ha'Chayim: Had he not known the hatred, he would have left openly. Had Hashem not told him, he would have thought how to find favor with him, and delayed there.
Why does it say that Yaakov heard the words of Lavan's sons, and he saw Lavan's face?
Malbim: (He explicitly heard Lavan's sons' gripes.) He saw Lavan's face, and understood that also he suspects Yaakov.
Lavan's sons were a three days' journey away, tending the animals that Lavan took (30:36). Yaakov heard others repeat their words. Lavan was nearby. (PF)