What is the ruling on imported meats and frozen chicken when the method of slaughter is unknown?

It is permissible to eat meats imported from the lands of the People of the Book, unless there is evidence of their unlawfulness, based on the Almighty’s saying: (This day [all] good foods have been made lawful for you. And the food of those who were given the Scripture is lawful for you and your food is lawful for them) Al-Ma'idah/5. The fact that some slaughterhouses perform non-Shar'i (un-Islamic) slaughter does not render all imported meats unlawful, because the default is permissibility until proven otherwise.