One of the big changes the 19th century brought in the wake of the French Revolution, was that we began to register everything. The concept of a census and people registers is an old one, but in modern history it became a standard operating measures of modern states. Everything is registered in modern states, when (and where) you are born, when (and where) you die, when (and who) you marry & everything in between and so much more.

So basically, you'd have an administrative footprint that allows you to compare.