From GpsWiki

The best way to check the accuracy of a GPS is to load the coordinates of a [benchmark] disk (a survey marker) whose location has been measured precisely (often to within a centimeter). It is important to use a benchmark whose horizontal location is adjusted, not scaled--scaled coordinates have been estimated from reading a map, and may be a hundred yards off.

A relatively easy way to find an adjusted location benchmark in your area is to search the [benchmark database] at geocaching.com using your zip code. This will yield a list of benchmarks within the zip code. Go down the list, checking only benchmark disks until you find something whose coordinates will be followed by a phrase such as "Coordinates may not be exact. Altitude is VERTCON and location is ADJUSTED." (emphasis added). The geocaching.com database is several years old, but it is easily searched and it has the occasional benefit of reports of [benchmarkers] who have posted the results of their efforts to find specific benchmarks.

Triangulation stations and traverse stations are good types to try; avoid bothering with icons that indicate the benchmark is missing ("?") or destroyed (broken pieces).