If the region where the meat mentioned is found has only People of the Book, namely Jews and Christians, then their meat is halaal, even if it is not known how they slaughtered it, because the basic principle is that meat slaughtered by them is halaal, because Allaah says (interpretation of the meaning):

“Made lawful to you this day are At‑Tayyibaat [all kinds of Halaal (lawful) foods, which Allaah has made lawful (meat of slaughtered eatable animals, milk products, fats, vegetables and fruits)]. The food (slaughtered cattle, eatable animals) of the people of the Scripture (Jews and Christians) is lawful to you and yours is lawful to them”

[al-Maa’idah 5:5].

If there are other kaafirs in the region, then do not eat it, because that means there is doubt as to whether it is halaal or haraam. Similarly if you know that those who sell these meats slaughter the animals in a way that is different from the shar’i method, such as strangling or electric shock, then do not eat it, whether the one who slaughtered it is a Muslim or a kaafir, because Allaah says:

“Forbidden to you (for food) are: Al‑Maitah (the dead animals — cattle — beast not slaughtered), blood, the flesh of swine, and that on which Allaah’s Name has not been mentioned while slaughtering (that which has been slaughtered as a sacrifice for others than Allaah, or has been slaughtered for idols) and that which has been killed by strangling, or by a violent blow, or by a headlong fall, or by the goring of horns — and that which has been (partly) eaten by a wild animal — unless you are able to slaughter it (before its death)”

[al-Maa’idah 5:3]. End quote. 

May Allaah help the Muslims to understand their religion, for He is All Hearing, Ever Near. End quote.