Like I think most countries the US does a survey to estimate the unemployment rate of the population. Here are more details from the US department of labor's site:
Where do the statistics come from?
Because unemployment insurance records, which many people think are the source of total unemployment data, relate only to persons who have applied for such benefits, and since it is impractical to actually count every unemployed person each month, the Government conducts a monthly sample survey called the Current Population Survey (CPS) to measure the extent of unemployment in the country. The CPS has been conducted in the United States every month since 1940 when it began as a Work Projects Administration project. It has been expanded and modified several times since then. As explained later, the CPS estimates, beginning in 1994, reflect the results of a major redesign of the survey.
What are the basic concepts of employment and unemployment?
The basic concepts involved in identifying the employed and unemployed are quite simple:
* People with jobs are employed. * People who are jobless, looking for jobs, and available for work are unemployed. * People who are neither employed nor unemployed are not in the labor force.
For more details you can check their web site.
As for the EU countries here is an earlier post on that question.
The reason I referred to Census data earlier is because if you want to get more detail on the unemployment situation you need census type data that is more detailed then the monthly surveys done to assess the level of unemployment. I'm not as familiar with the specific questions asked in the monthly surveys of different countries whereas I've worked with census data from several countries to look at labor statistics in context.
People who are jobless, looking for jobs, and available for work are unemployed.
I argue below that this would ideally be corrected to make an allowance for the productive nature of a reasonably brief and effective search for a good job. At present, some "unemployment" corresponds to a necessary and productive form of labor, which makes the definition perverse. Words and ideas I offer here may be used freely and without attribution.