What is the ruling on eating meat from restaurants in foreign countries without knowing its source, how it was slaughtered, or the religion of those who slaughtered it?

If it is possible to verify that the slaughterer is a person of the Book and that the death occurred through a lawful slaughter (dhakāt shar‘īyah) by cutting the trachea and the two jugular veins, and not by stunning or strangulation, then there is no objection to eating from these meats. Otherwise, they must be avoided, because the default status of slaughtered animals is حرمة (prohibition) until their permissibility is known.