How can one verify whether a father's spending comes from lawful or unlawful money? What is the ruling on spending from usurious money in the Qur’an and Sunnah, and is it permissible to refuse to accept maintenance from him?

If your father's wealth is mixed, stemming from both usurious transactions and permissible earnings, then there is no harm in benefiting from it, so long as you do not know for certain that the specific money you are using is the unlawful money itself. If all his wealth were from usury, and you were compelled to take from it, then there is no sin upon you until you can do without it by acquiring lawful wealth. It is advisable to gently and wisely counsel your father to desist from usurious transactions.