The job is not finished when the last panel goes on. It is finished when the system has run, the paperwork is in your hand, and you know where everything shuts off. Stay in the room while the crew is still there and walk the work with them. This is the easiest moment to ask questions, because the people who did the work are still on site.
Watch a full run before anyone leaves
Ask for the system to run through a complete cycle while you watch. For a furnace, that means a call for heat and the burners lighting. For an air conditioner or heat pump, that means the outdoor unit starting and the indoor air changing temperature within the expected time. For a dishwasher, a garbage disposal, or a clothes washer, it means a full cycle with water going in and draining out where it should. Look for drips at every joint, listen for rattles that were not there before, and note anything that feels off. A short test tells you little. Ask the crew to run it long enough to reach steady operation.
Find every shutoff and have it named out loud
Ask the installer to show you each shutoff the new work depends on: the water valve, the gas valve, the breaker or disconnect, the condensate drain outlet, and any service switch. Have them name each one while you stand next to it. Then confirm that you can reach it yourself. A valve behind a new cabinet is not much use if nobody can get to it.
Check the labels and the tags
Most equipment carries a data plate with a model number and a serial number. The serial often encodes the manufacturing date, though the code format varies by maker, so check the manufacturer's own decoding guidance rather than guessing. Photograph the data plate before the crew leaves. Confirm that the breaker or fuse is labeled for the new circuit. Ask where the filter size is marked, on the unit or on the return grille.
Read the paperwork while they are still there
Ask for the invoice, the installation record, and any startup or commissioning sheet the crew filled out. The record should name the equipment, the job, and the company that did the work. If a permit was required, ask for the permit reference. Ask what the manufacturer warranty covers, who holds it, and how to start a claim. If a service agreement was offered at the same visit, treat it as a separate document. Look for where the terms that set coverage start, and where cancellation and transfer are described. Do not sign a service contract at the door without reading its definitions and its waiting period, if it has one.
Write down what you will need later
Put the install year, the model, the serial, and the crew's contact details in a single place. Keep the photos with the paperwork. When a planning year matters later, you will want the install year on record rather than an estimate. The system pages on this site turn an install year into a planning window, such as the gas furnace guide or the heat pump guide, depending on what you had installed.
Questions to ask before the truck leaves
Ask the crew to answer these in person. What did you test? Where are the shutoffs? What should I watch for in the first few days? Who do I call, and which number do you want me to use? What is outside the scope of your work? Write the answers on your copy of the paperwork.