I am a Cohen from Brazil, with the Cohanim tradition, as my father and grandfather. My mother,as my grandmother is a Jew. We are very proud of that. However, I can check my lineage for 5 generations only. Since there is no way to check and verify 100% of the whole lineages and generations of the Cohanim today, since the Temple Period. How can I, or any Cohen, be absolutely sure, or even being able to really prove with documents or ketubahs, that there were no past "forbidden halacha mistakes" commited by any of our Cohanim ancestors? How can I , or any other Cohen in the world, be absolutely sure that our Cohen ancestors did not married a divorce or a convert?
There is much discussion about this question in the Gemara and the Halachic authorities. Today being a Cohen entitles one only to Duchen and to receive the first Aliyah, both relatively low-risk problems from a Halachic perspective. Therefore, the Rama rules that we allow someone to do either based on his say so that he believes himself to be a Kosher Cohen. When the Bet Hamikdash is rebuilt, soon and in our days, Eliyahu will come and tell us which Cohanim indeed have 100% pure pedigree and can perform the service.