An alarm that has been silent for a long time is not the same as an alarm that works. In a house where someone is home all day and night, the units are easy to forget, because they sit quietly on the ceiling until the one day they are needed. Keeping them working is an ongoing job, and it helps to decide now who owns each part of it.
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Count the units and name each one
Walk the house and list every smoke alarm and every carbon monoxide alarm, including any combined units. Note the room and the floor for each. Then write one person's name beside each unit. A unit with no name beside it will be checked by no one. Many families find that the person awake most of the night should own the units in the sickroom and the hallway, while another adult owns the rest.
Read the date on the back
Most alarms have a manufacturer's date printed on the back or the side. That date tells you how old the unit is and when the maker says it should be replaced. Write the date you found on a card, and write the replacement date on the same card. If a unit is past its date, replace it rather than trusting it, even if it still makes a sound when tested.
Test on a fixed day
Pick a day each month and test every unit with its test button. Press it and listen for the full alarm. Write the test on the card with the initials of the person who ran it. If a unit does not sound, check its batteries first, then replace it. A chirp that repeats every so often is usually a sign the battery needs changing, and that chirp should be dealt with the same day, not the next weekend.
Batteries and the sound of a warning
Decide who buys batteries and where they are kept. Keep a spare set in the same drawer as the instruction sheet, so the person doing the test does not have to hunt for them. When a unit starts to chirp at night, decide in advance who gets up to deal with it. A chirp at three in the morning is hard to ignore and easy to put off, so the owner of that unit needs a plan for it.
Check the rules where you live
Local fire code and the fire department in your area set the specific rules for how many units a home should have and where they go. Those rules differ from place to place, so check them with your local fire department or code office rather than relying on a general list. The Sources page lists the places this site draws on for general information.
Keep the card where it can be found
Put the card with the unit list, the owners, the dates, and the test log in one place near the front door or in the kitchen drawer with the batteries. When the owners change, when a unit is replaced, or when a test fails, update the card the same day. A card that is kept up is the proof that someone is still watching the ceiling.