What really was behind Lavan's introductory statement, "If I have now found favor in your eyes"?
Seforno: What Lavan really meant to say was, 'If you love me (which, bearing in mind our family relationship you really should), then it is not right that you should leave me.' 1
What he had in mind of course, was that the Berachah that Yaakov brought with him, should continue unabated.
What did Lavan mean when he said "Nichashti... "?
Ramban #1, Targum Onkelos and Seforno #1: He had arrived at that conclusion by trial, based on the fact that, since Yaakov's arrival, his business-ventures had flourished way above normal, a clear indication of Yaakov's righteousness. 1
Rashi, Ramban #2, Seforno #2 and Targum Yonasan: He meant that, using divination, 2 he had discovered that the Berachos that he had enjoyed since Yaakov arrived by him, 3 were in the merit of Yaakov. 4
Malbim: All my Berachah is due to you. If you will leave, I will return to be poor. I did not give my daughters on such condition. This is like a buyer who throws the coins [paid for the sale item] to the sea. He nullifies the sale! 5
Ha'amek Davar: Do not claim much for your work. The Brachah was due to you, and not due to your hard work!
As the Gemara says, 'With a Talmid-Chacham comes Berachah' (Berachos 42a).
A form of witchcraft based on the twittering of birds (Ramban, Parshas Shoftim).
Even if he would return to be poor, Lavan enjoyed 14 years of prosperity in exchange for his daughters, and now he has sons in place of them. Why should the sale be Batel?! (PF)
How can one reconcile Lavan's apparent belief in Hashem, with his statement later on, "Lamah Ganavta es Elohai" (31:30)?
Oznayim la'Torah #1: He was quite happy to believe in Hashem if it was to flatter Yaakov.
Oznayim la'Torah #2: He believed in Hashem be'Shutfus - in partnership with idolatry.
QUESTIONS ON RASHI
Rashi writes: "I ascertained using divination, that I was blessed due to you... now I have sons." Perhaps Lavan referred to the increase in his flocks?
Gur Aryeh #1: Lavan had flocks even prior to Yaakov's arrival, only that now there were many.
Gur Aryeh #2: Divination is relevant when something is not patently obvious (i.e. determining the cause of an unexpected effect - (EK)). Since Yaakov was working with the flocks directly, as a shepherd, it would not be divination to attribute their increase to him! But the sons born to Lavan were seemingly unrelated to Yaakov's arrival.
Rashi writes: "[Lavan said,] when you arrived, I had no sons, as in the verse, 'Behold, his daughter Rachel is coming with the sheep' (29:6); if he had sons, would he have sent out his daughter among the shepherds?!" But perhaps he had sons who were still little; or there were so many flocks that his daughters helped as well?
Gur Aryeh: Earlier, the Torah emphasizes, "For she was a shepherdess" (29:9). Rachel did not go out by chance occasion; rather this was her regular job. It must be telling us that Lavan had no sons.