How can one distinguish between lawful and unlawful money when they are mixed, and does all the money become unlawful if a part of it mixes with unlawful money?

If a person knows the amount of unlawful money, he must dispose of it, and the rest of his money is lawful. It was reported from Ibn Taymiyyah that if unlawfully acquired money is mixed with lawful money, then what is obligatory is to remove the unlawful portion, and the remainder becomes lawful. If the matter is unclear and he does not know how much is unlawful, then he must investigate until he is certain that what remains is rightfully his. The prohibition pertains to the liability of the one who takes the unlawful money, not to the physical coins themselves.