The easiest way to find out my nationality - ask the parents what nationality they are. If the father and mother belong to the same nation, for example, both Tatars, then your nationality is Tatar too. In the case when parents of different nationalities, you need to determine your individually, depending on the traditions of a people. So, the Russian nationality is determined by the father, the Jewish mother. So if your father is Russian and her mother is Jewish, in Russia, you're Russian, and Israel - Jew. And Vice versa.
For those who don't know, for some reason of the nationality of their parents (for example, a person brought up in an orphanage or was adopted), there is a common criterion. Belonging to a nation is determined not by place of residence, and follow the traditions or customs of a people and, most importantly, knowledge of the language perfectly. For example, you live in Russia, but my native language is you - Bashkir, you are a devout Muslim, circumcised, honor the Bashkir ceremonies. So, you are the Bashkirs.
There are certain physiological signs: the skull structure, eye shape, skin color, hair, type of body hair growth in which man can be attributed to one or another race. However, if you've got a lot of different nationalities that often (for example: his own father was Russian and Jewish relatives in the mother - Ukrainians and poles), to determine with precision the nationality of appearance will be difficult. In this case, you can choose the nation, which suits you the most. Or, like many, just to be international.
To change nationality, some peoples need to go through a certain ritual. So that a non-Jew to become a Jew, will require circumcised men convert to Judaism for women and also test the knowledge of the laws of the Torah.