Rashi citing Shabbos 113b #1: He saw Chachmah in her. She would take two sheaves (of Leket, i.e. what falls during harvesting), but not three. (Three are not considered Leket.)

Rashi citing Shabbos 113b #2: He saw Tzeni'us in her. She would gather attached [Shichchah] while standing, but she would sit to pick up from the ground, to avoid bending over.

Vilna Gaon: Since they told him that she is proper for him (refer to 2:4:3:1 and the note there), he needed to ask about her family.

Malbim: He saw that she was alone, and gathered about an Eifah (17), a great amount. He thought that she is the wife or daughter of a harvester, who expelled other Aniyim so only she would gather about him, and she gathered also standing grain (not only Leket). "L'Mi ha'Na'arah ha'Zos" - of which harvester is she?