You want one answer to one question: is there an alarm in the right place, and does it work? Getting that answer takes about four steps, and most of the trouble comes from skipping the paperwork in the middle. This is the install and service path from the box to the next visit, written so you can hand it to whoever is standing in the hallway.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
Start with the box, not the store shelf
Before anything goes on a wall, read the instructions printed on the packaging or the manual that comes with the unit. Those papers decide the things you can't guess from the outside: how high the unit mounts, how far it should sit from fuel-burning appliances, whether it needs to sit on each level, and whether it is a plug-in, a battery model, or a sealed unit with a built-in power supply. Take a photo of the instructions and save it in the folder with the receipt. You will want that photo the next time someone asks what kind of unit is up there.
Also look for the end-of-life marking. Most units print a date or a timer on the back or on the cover. Copy that down exactly as printed, then write the replacement point on the inside of the cover or on a card taped to the wall nearby. Your memory will not survive a year of visits. The label will.
Choose the spot with the house in front of you
Walk the house with the instructions in hand. Stand where the device will sit and ask three plain questions. Is it on each level where people sleep or pass through? Is it far enough from the kitchen, the furnace closet, and any gas appliance that it won't be bumped or blocked every day? Is there a clear path to the spot for someone who needs to reach it without a ladder? If the answer to the third question is no, move the spot to where it can be reached, as long as the instructions still allow it.
Do not mount it inside a closet, behind a curtain, or on a shelf where a stack of boxes will eventually sit against it. A unit that can't be seen or heard is not doing its job, however well it was installed.
Mount, then test before you close the wall
Attach the mounting plate first and level it. Snap or screw the unit onto the plate, following the order printed on the box. Once it is on the wall, press the test button and wait for the full sound pattern to finish. Do not count a short chirp as a pass. If the unit has a separate reset or silence button, press that once to confirm it responds. If it is a set of interconnected units, test each one, since one that is not connected can leave a room silent while the rest sound off.
If the test fails, stop. Check the battery or power connection, confirm the unit is seated fully on the plate, and read the troubleshooting page in the box. Do not wedge the unit with tape or remove the battery to make a noise stop. Call the manufacturer's line listed on the packaging if the test still fails.
Log the install so the next visit has a starting point
Write down the date you installed it, the location, the model name, the end-of-life marking, and the test result. Put that note in the folder with the receipt and the instruction photo. If you are the one far away, send the same note to the person who is local, so there is one version everyone can read. Ask them to tell you the next time they press the test button and what they heard.
On each visit after that, press the test button, look at the wall around the unit for dust or damage, and compare the end-of-life marking against the card you wrote. This takes less than a minute. It is the difference between knowing the unit works and assuming it does.
What to do tonight
Find the end-of-life marking on every alarm in the house and write it down. Press each test button. If any unit is missing, check whether the house has a fuel-burning appliance, a garage attached to the living space, or a fireplace, since those are the usual reasons a home needs one. Put the list in the folder and text it to the person who is local.
What to leave with the fire department or the doctor
If you are not sure the placement meets the instructions, ask the local fire department whether they offer a home safety check or can point you to one. Many departments will look at a placement question at no cost, and they will tell you whether the spot works. If anyone in the house has headaches, dizziness, or nausea that seem to ease when they leave the home, tell the doctor about it at the next appointment and say when it happens, so the doctor can decide what to look at.
For the wider set of local help that a St. Louis family can use, the Local Help for a St. Louis Parent page lists the kinds of people to call and what to ask them.