The Gemara draws a distinction between the status of a Mamzer and that of a Chalal. A Mamzer is a child of a forbidden union, such as if a married woman had an adulterous relationship. The child from this union has the permanent status of Mamzer, which forbids him or her from marrying (though a Mamzer and a Mamzeret may marry one another). The term “Chalal” refers to the product of a union between a Kohen and a woman forbidden only to Kohanim. For example, if a Kohen marries a divorcee, in violation of the Torah prohibition, the product of this marriage is called a Chalal, or a Chalala in the case of a girl. A chalal is not considered a Kohen, even though his father is a Kohen and his last name is “Cohen.” A chalala may not marry a Kohen, even though her father is a Kohen, since she was born from a relationship that violated the laws of the Kohanim. Unlike a Mamzeret, however, she may marry non-Kohanim.
The Gemara states that “Jews recognize the Mamzerim among them, but do not recognize the Chalalim among them.” This means that the phenomenon of Mamzer is generally widely publicized and when there is a Mamzer, people know about it. Therefore, when two people decide to marry, it is not necessary for each to do thorough investigations to ensure that the other is not a Mamzer or Mamzeret. The status of Mamzerut is well-known and therefore in the absence of any particular reason to suspect that somebody has this status, there is no need to thoroughly investigate a potential spouse’s family background to check for “Mamzerut.”
The status of “Chalalut,” however, is not widely known. People are not necessarily aware when a Kohen marries someone forbidden to him and it is therefore possible for a person to be a Chalal or Chalala without it being public knowledge. Therefore, we cannot automatically trust a person who claims to be a Kohen. Of course, if one comes from a family that is well-established as proper Kohanim, then we certainly treat him as a Kohen. But if a person is not from one’s own town and no one in the community knows him, we cannot automatically accept his claim of being a Kohen.
The Shulchan Aruch states that the individual in such a case should not be given the first Aliya and should not recite Birkat Kohanim – and certainly would not be given Teruma in the days when Kohanim received Teruma – until his background is investigated and he is determined to be a proper Kohen. Since the status of “Chalalut” is not widely publicized, a person’s claim to be a proper Kohen cannot be accepted without some research into his background.
The Rama (a”h) disagrees with the ruling of the Shulchan Aruch. He claims that in our day and age, we can accept a person’s claim that he is a Kohen and call him to the Torah as a Kohen. From the Shulchan Aruch, however, as mentioned, it emerges that we cannot trust a person’s claim even with respect to the Aliya to the Torah. Therefore, if a person moves into the community and claims to be a Kohen, his background should be checked before he is treated as a Kohen.