Depends on the country. México for example is quite easy.

>>18730580
Depends on a country and particular situation.
For instance, modern day Poland you can basically assume that if someone's grandparents were non-naturalised Polish citizens then you're like 99,5% accurate, basically missing a couple thousand jews that didn't go to Israel in 1968 and similar amounts of eastern slavs plus some random Armenians and Tartars.
So a legislation would be really easy. Just denaturalise everyone who doesn't fit that description and then only grant citizenship to people who have 2 Polish parents gg no re.

France is going to be basically impossible because they were getting mutted on mass scale since 1789 and that's when the paper trail starts becoming completely worthless as well(you're French if your mom plopped you out in france hurr durr).
In Germany the Polish method would work reasonably well, you'd just have to adjust for some slavs living there and maybe push the timeline a little bit back.
In the UK it's really easy you just have to denaturalise working class(mutted with the irish) and boat niggers and you basically get 90% English 9% scotts and the rest is some strangely successful irishmen(probably frauds but too few of them to bother) and people who had some bastard indian blood in them like Kitty Kirkpatrick.

As for ethnogenesis I think it becomes even more convoluted than that. The most obvious things you can track is:
>When did other people started to refer to them as <national>
For instance, in 16th century everyone called Spaniards Spaniards, Catalan/Aragonian/Castillian autism people from outside of Spain didn't care about.
>When did they start exhibiting an in-group out-group dynamic
Usually it'll be rather negative stuff like pogroms but they show a formed identity, for instance pogroms of Germans in 13th century Krakow.

>>18730593
How do you define a Mexican?

For starters, even before the conquest, Mesoamerica was a hodgepot of different peoples, including Toltecs, Chichimecas, Mayas, Zapotecs, Mixtecs and Yaquis, most of which didn't have anything to do with Aztecs besides maybe giving them tribute.
During the Viceroyalty, you got the Mestizaje, as well as the arrival of Spanish criollos and a tiny amount of blacks, but even then, 60% of the country remained native at independence
After Independence, Mexico recieved many small but notable immigrant waves from several parts of the world, including French, German, Irish, Chinese, Lebanese, Jews (the current President is even one)
The following the Revolution, the state official ideology promoted Mestizaje as the official ethnos of the Mexican people (But ONLY between Spanish and the Aztecs, everyone else got quietly lumped together into Mestizo as well)
The only real hard line, is that being born in the US is a big no no (Which is why Chicanos aren't seen as Mexican by Mexicans)

>>18732143
The ethnogenesis of Mexico begins with the first whiteman to creampie an Indian.