There are two separate answers depending on whether the building has a fire alarm system or domestic alarms, and both rest on the same principle: the equipment is tested often enough that a failure is found by the test rather than by a fire. For a workplace system, that means a weekly test by the occupier and a periodic service by a competent engineer.
How to test fire alarm with key: the weekly workplace test
A different manual call point is operated each week using the test key, and the alarm is confirmed to sound throughout the building. Rotating the call point means every one is proved over a period rather than one being proved fifty times. The test is recorded in the fire log book with the call point tested, the date, and whether the alarm sounded correctly. It is also the moment to confirm that held open doors released.
The periodic service
A competent engineer inspects and tests the system on a schedule, typically in visits spread across the year so that every device is functionally tested over an annual cycle, with the panel, the batteries and the cause and effect checked. That is a different activity from the weekly test and neither substitutes for the other.
Domestic alarms
Press the test button weekly on one unit and confirm that every interlinked unit sounds. Clean them periodically as the manufacturer directs, because dust in an optical chamber causes both missed detection and nuisance alarms. Replace units at the end of their stated life from the date of manufacture rather than from the date of installation.
Why the record is the point
The maintenance duty is continuing and is evidenced by the record. A log showing a test every week for two years, with the occasional failure and what was done about it, is far stronger evidence than a service certificate alone, because it shows the system was being watched by the people in the building.
How often should smoke alarms be tested, at home and at work
At home the answer is weekly, with the button, which tests the sounder and the electronics rather than the sensor, plus a battery change annually on any unit that still takes one and a clean with a vacuum a couple of times a year. Sealed ten year units still need the weekly press. At work the interval is the same but the method is different: on a system with call points, one point is tested each week and the tested point rotates so that every one is proved over a period, with the result written in the log book, and a competent engineer services the system at least every six months. In both cases the interval that actually gets missed is the annual one: a domestic alarm's ten year replacement date and, on a system, the full annual inspection and the battery capacity check. Test after the clocks change if a weekly habit has never stuck; it is worse than weekly and much better than never.
Other questions people ask
Does the weekly test have to be at the same time?
A consistent time helps occupants distinguish a test from a real alarm, and many buildings publish it. What matters is that it happens and is recorded.
Who can do the weekly test?
Anybody trained to do it, which is a short briefing. It does not need an engineer.
Should the fire service be told before a test?
If the system is monitored by an alarm receiving centre, they must be notified before the test and told when it is finished, or the test becomes a call out.