What is the ruling on eating fried foods (potatoes and other dishes) in non-Muslim countries, given the possibility that they are cooked with oils or fats derived from pork fat, and is it obligatory to inquire in every restaurant, or is it sufficient to say Bismillah and eat as long as there is no evidence of that?

If there is no reason to suspect the oil used for frying potatoes, then the default ruling is that it is permissible unless proven otherwise. There is no harm in consuming foods fried in oil of unknown origin, unless it is proven that it is derived from a forbidden animal or an animal that was not slaughtered according to Islamic law. It is permissible to ask the restaurant owner about the source of the oil.