Kesubos 25b - Who is a Kohen?

It is clear from the mishna 23b that one is not beleived to establish himself as a kohen. One would require eidim to verify that he is a kohen to receive any privellages that are associated with being a kohen. Since the privileges of Teruma, Nisus Kapayim, and Yuchsin are given specifically to one who proves that he is in fact a kohen, the gemara discusses the possibility of being machzik from one to the other i.e. whether a kohen for one thing is a kohen for everything. The gemara says that even reading from the Torah first as a kohen, would be proof that he is a kohen for other things also which clearly indicates that we don't allow someone to be called to the torah as a kohen unless he can prove it. The Rambam and Shulchan Aruch E.H 3:1 say exactly that - if one claims that he is a kohen but cannot prove it, he is not entitled to any benefits of a kohen, but he automatically makes himself forbidden to a divorced woman and from entering a cemetery based on the concept of "shavya anafshay chaticha d'issura". However, the Rama follows the opinion of the Ran that since nowadays there is no concern of leading to eating Teruma (since there is no Teruma d'oraysa), we are lenient to allow one to read from the torah as a kohen by simply claiming to be a kohen. The Rama extends this to giving the Birchas Kohanim (duchening) as well. The Beis Shmuel challenges the addition of the Rama, although we trust him to allow him to read from the torah, that is because there is no issur involved, but we cannot allow him to duchen since it is assur for a non-kohen to duchan? The Beis Shmuel answers that the reason one is not believed to say he is a kohen is because he gets real benefit from being a kohen i.e. free teruma, but nowadays where there is no benefit with being a kohen, he is believed m'ikar hadin. Therefore even if it affects issur such as duchening, he is believed.

It comes out a nafka minah between the rama and shulchan aruch, why one who claims to be a kohen is assur to marry a divorced woman. According to S.A. it is because of "shvya anafshei", but acc. to rama he has ne'emanus. One way or the other, one who has a good amasla such as new information that he has regarding his yichus should be believed to annul his kohen status even after he has read as a kohen and duchened for many years (since we see in the gemara 22a that amasla works even to override shavya anafshei).