Q. I received a bottle of whiskey as a gift from a non-religious Jew. Do I need to be concerned that it was possibly owned by him during Pesach (e.g., maybe he keeps whiskey in his house year round and when the need arises gives it to someone as a gift, or maybe he purchased the whiskey from a Jewish-owned store that owned it during Pesach? Does it make a difference when I received it (in December or in April)? It is not possible to ask him where he got it from without embarrassing him.

A. The prohibition of chometz after Pesach is Rabbinical. Mishna Berura (449: 5) mentions that Poskim disagree on sofek or doubtful chometz that should in principle be permitted as other doubtful Rabbinical prohibitions are, if that includes eating or only deriving benefit. He mentions that even the opinions that prohibit eating, it is only immediately after Pesach, when it is likely that it was on a Jewish domain during the Yom Tov.
Horav Shlomo Miller’s Shlit’a opinion is that in your case there may be other additional doubts, since some non-religious individuals also sell their chometz or are included in the sale by others. Therefore the whiskey is permitted.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a