Reference: Fataawa al-Mu’aasirah – Page 251
Question: In most of the restaurants in the West, they fry the fish and the potatoes (chips) in a big [deep] fryer filled with oil.
1) Is it permissible for us to eat fish or chips fried in oil which has been used to fry non-halaal meat, or possibly even pig meat?
2) Is it obligatory upon us to ask the owner [or employees] of the restaurant whether they have fried anything else in that oil or not? Or should we [just] eat [the fish or chips] without question?
This is a daily problem the Muslims who live in the West face, and may Allaah reward you with good.
Response: So long as we know that most of what is fried [with this oil] is impure [for the Muslims to eat] from dead [meat (of the animal which had died prior to slaughter)] or pig [meat], then it is imperative that we ask. However, if we do not know whether most of what is fried [with this oil] is impure [for the Muslims to eat] or other than that, then it is not obligatory to question. And Allaah [alone] has the Complete knowledge [of all affairs].