How do I know that someone has emigrated? Or is there any way to find out?

Unless you know the unique ID number issued by the immigration office of the country where the immigrants live, you can only make inquiries through an intermediary.

However, the personal information of the immigrant is strictly confidential, that is to say, unless he or she voluntarily admits it, there is no better way to inquire.

If they are just immigrants, but not naturalized, they will still retain their household registration and nationality in China.

There is no conflict at all. Immigration is not equal to naturalization.