Batteries can die without you knowing it. It's best to check smoke detector and carbon monoxide detector batteries at least twice a year. In fact, some recommend replacing these batteries twice per year. I find that overkill. The batteries in my detectors seem to last two to three years before I get that annoying beep every minute. But it is good to check the batteries anyway.
How do you check a battery?
The best way is to have one of these battery load testers. These testers actually put a load on the battery when they measure the battery capacity. That means the accuracy of the reading is much better than just measuring the volts with a standard VOM (volt-ohm meter).
I try to check my batteries at least once per year, and hopefully twice per year using a load tester. I replace them if they are at 20% or less.