According to the NOAA's [http://www.ngdc.noaa.gov/seg/geomag/declination.shtml declination website], "magnetic declination, sometimes called magnetic variation, is the angle between magnetic north and true north. Declination is considered positive east of true north and negative when west". The site has an [http://www.ngdc.noaa.gov/seg/geomag/img/us_dec_8x11.pdf Adobe map] of the U.S. (click the 'rotate clockwise' tool) that shows the general pattern of declinations in the U.S. For a specific declination such as the declination of the place where you want to go on a benchmark hunting expedition, use that site's "Compute your declination" link.
For benchmark hunting, you need to be able to use your magnetic compass to find ["true North"] ["azimuth"]s. The way to do this is to convert the true North azimuth to a magnetic azimuth. To convert a true North azimuth to a magnetic compass reading, subtract the declination value from the true North azimuth's value.
Example1: the datasheet information for a survey station in central Colorado, where the declination is +10, says that the azimuth from the station to one of the reference marks is 133 degrees. When you're at the station, on your compass, look in the direction of 133 - (+10) = 123 degrees to find the reference mark.
Example2: the datasheet information for a survey station in western Pennsylvania, where the declination is -10, says that the azimuth from the station to one of the reference marks is 133 degrees. When you're at the station, on your compass, look in the direction of 133 - (-10) = 143 degrees to find the reference mark.
Another way to find the declination is to read the declination value on a [http://www.topozone.com/ topozone] map. The value is below the bottom right corner of the map and is the value next to "M=". Be sure to pay attention to whether the magnetic declination value is + or - !