"Portable fire extinguishers must be tested periodically to ensure that they will withstand the internal pressure from the fire extinguishing agent and/or expellant gas. (See Coffee Break Training FP [Fire Protection]-2010-9 for test frequency.) Once the test is completed successfully, the results must be marked on the fire extinguisher for ready reference. Depending upon the type of cylinder, the record may be stamped on a high-pressure cylinder or have a permanent label applied to a low-pressure cylinder. (See Coffee Break Training FP-2010-3 for an explanation of the different cylinder types.)"
Coffee Break Training - Fire Protection Series No. FP-2014-7
United States Fire Administration: http://www.usfa.fema.gov/