If you're wondering whether the long black sedan with blacked-out windows you've seen around town belongs to a government official, the quickest way to know for sure is to check out the car's license plate. Like civilian license plates issued by a state's DMV, federal license plates use both letters and numbers, but government license plate codes also include lettering that stands for a particular government agency.
TL;DR (Too Long; Didn't Read)
A modern U.S. government license plate has "U.S. Government" printed along the top, but you can look for other clues that it is a license plate on a vehicle used by a government official and not a civilian.
History of Government License Plates
In 1927, letter prefixes began to be used on District of Columbia (as Washington D.C. was then known) license plates, with the letter "A" restricted to plates used by the district and federal governments. A few other states also issued special plates for federal government vehicles. A uniform license plate design for federal government plates was adopted in 1942. Before this time, plates on vehicles used by federal government agencies were obtained independently of other agencies or a central federal vehicle registration office. From 1942, some plates used letter prefixes to indicate the agency (such as "AF" for Air Force and "SAA" for U.S. Senate), a system still used today.
Government License Plate Style
The current style of flat license plates made by the federal prison system's correctional program uses Digital License Plate technology, first introduced in 2001. A later variation of this plate was made in 2009. On both styles, the size of registration number characters varies depending on the format. On the 2001 style, the American flag background design is blue only, and the plates have a blue border and no expiration date. On the 2009 style, the design is red and blue, and the plates have a red border and an expiration date, which is eight years from the date of issue.
Government License Plate Numbering
All federal plates are marked with a letter prefix to indicate the agency to which the vehicle was assigned, for example "C" (Department of Commerce), "DHS" (Department of Homeland Security), "LB" (Legislative Branch) and "VA" (Department of Veterans Affairs). Most plates are G-series plates assigned by the General Services Administration, which include a numeric code following the "G" prefix, to signal the vehicle type. For example, "G10" is a compact sedan, "G31" is an ambulance, "G71" is a medium truck, cab and chassis and "G90" is special equipment.