To confirm whether Su Chengyu and his group are from the Bei Tang Family, the simplest way is to directly contact the Bei Tang Family of Xia Country.
As the largest merchant city between Bei Luo Empire and Xia Country, there are naturally exchanges between Linbei City and the Bei Tang Family, even the Duke of Night Raven herself has made several deals with the Bei Tang Family.
But the Duke of Night Raven will not take the initiative to inquire with the Bei Tang Family, since she and Su Chengyu are currently in complete opposition. Once she contacts the Bei Tang Family and receives a positive response, her conflict with Su Chengyu would escalate from a personal level to a height involving the entire Bei Tang Family.
Before asking, the Duke of Night Raven could explain herself as being unaware, but if the Bei Tang Family side has already received the news, and she is still unwilling to let go of Su Chengyu, that would be a provocation to the entire Bei Tang Family.