What is the ruling on renting a shop from a doctor who built commercial shops on land originally designated for a medical clinic, which he obtained at a symbolic price, knowing that the renter is not certain of the doctor's violation of building conditions, and is it obligatory for him to verify that?

You are not obliged to search for the origin of the land. Rather, the matter is built upon the default principle, which is the permissibility of the wealth. So, if you do not know the status of the wealth in the Muslim's possession, then base the matter on the default principle. If the man is known to have unlawful wealth, then there is a difference of opinion among scholars regarding this. As for the discreet Muslim, there is no doubt in dealing with him in the first place.