We do not eat the meat of a person, nor marry them, unless we KNOW that the person is either a Muslim or Ahl ul-kitab. However, I've heard an opinion that is somewhat more lenient. This opinion (I don't have a reference, it was mentioned in passing by a sheikh) says that if you KNOW the animal was slaughtered (for example, you see that the neck has been cut in the proper way), but do not know if the person who did it was a Muslim/kitabi or not, then you may eat the meat IF you are in a town where the majority of the people are KNOWN to be Muslims/kitabis. But I don't have any more detail than this.