How on earth do people know what specific counties they have been to?
It depends for the country. For the US it is either odd or mighty difficult because there's the common obsession with driving extremely long distances to one destination or one area. However, if you've been rather thorough or regional in your travels you can easily go through your memory and just ask yourself if you've been to a place and then compare it to a list of counties.
Here's mine revamped with places I've lived, been to and only travelled through. There's somewhere in Eastern Québec that I've camped at a few times but I don't remember where. I've never been on more than a 3 seater plane and never been out of the country. I just asked myself of the places I have experiences being, then checked roads for a few trips.