We have people who

we know their family history.

So the way we know whether someone married a cousin or not is not by asking them in a survey, but it's by looking at their actual family tree so we can observe who your grandparents were.

And if you and your spouse share some grandparents, then you're cousins.

First cousins, if you share two grandparents.

And...

And then we could observe something about the kids.

But in many societies that it's almost a death sentence.