A date is stamped on the neck/bottom of the pressurized dry chemical extinguisher. i.e 04 - could mean 2004 or 1994, but nonetheless, the 4th year. It could be on a label, but should be...
In the US, most state laws and the NFPA (national fire protection agency) require annual testing.
Another view:
Actually, TESTING is not required by any rules, annually, for portable fire...