Based on Rashi's own words, and based on Rashi's sources, he did not know for certain that Lavan was a deceiver, but knew this as a cultural aspect of the people in Lavan's place, which is, then, perhaps something well known.
From Rashi's own words, we might see some uncertainty as to Lavan's status as deceiver or righteous person. On verse 12, Rashi writes:
כי אחי אביה הוא: קרוב לאביה, כמו (לעיל יג ח) אנשים אחים אנחנו. ומדרשו אם לרמאות הוא בא גם אני אחיו ברמאות, ואם אדם כשר הוא, גם אני בן רבקה אחותו הכשרה:
that he was her father’s kinsman: Heb. אִחִי אָבִיהָ, lit., her father’s brother. Related to her father, as (above 13:8):“we are kinsmen (אַחִים)” (Pirkei d’Rabbi Eliezer , ch. 36). Its midrashic interpretation is: If he (Laban) comes to deceive me, I, too, am his brother in deception, and if he is an honest man, I, too, am the son of his honest sister Rebecca. [from Gen. Rabbah 70:13]
Throughout, Rashi channels Bereishit Rabba. The midrash referred to in the question, in Midrash Rabba, reads as follows:
ויאהב יעקב את רחל
בגין דאנא ידע דאנשי מקומך רמאין, לפיכך אני מברר עסקי עמך.
"And Yaakov loved Rachel... [and he said, "I will work for you seven years for Rachel, your younger daughter.]Since I know that the people of you place are tricksters, therefore I will make my matters clear with you."
Thus, in the original midrash, Yaakov explicitly tells Lavan that he knows that the people in his location are liars.
This information need not have come from his mother. If it is a cultural trait, then this could have simply been well known information to any adult living in the Ancient Near East.
