Is it permissible to eat fish or french fries cooked in oil that was used to fry non-halal meat (such as unslaughtered meat or pork), and is it obligatory to ask about that?

As long as we know that most things fried in it are impure, like carrion or pork, then we must ask. But if we do not know whether most things fried in it are impure or otherwise, then we are not obligated to ask.