Start with how they measure
Good installation begins before installation day. Ask who measures your openings and how: a salesperson estimating from the driveway is not the same as a technician measuring each opening to the millimetre. Mis-measured windows cause the delays and filler panels that ruin an otherwise good project.
Ask what happens if a unit arrives the wrong size. The answer reveals whether the company controls its own ordering or is at the mercy of a supplier.
Pin down the installation method
For each opening, ask whether they recommend retrofit or full-frame replacement and why. Retrofit is faster and cheaper but leaves the old frame; full-frame costs more and disturbs more trim but lets the installer address rot and insulation properly. There is no universally right answer — there is a right answer for your house, and a good installer can explain it opening by opening.
Then ask about the details that determine performance: how they flash the opening, what insulation goes around the frame, and how they air-seal. These answers separate installers who understand building science from those who just set windows.
Ask about the crew and the day itself
Who will actually do the work — employees or subcontractors? How many windows do they install per day, and how many days will your project take? How do they protect floors and furniture? Where does the old material go?
Also ask what a typical day looks like: start time, how openings are secured overnight if the job runs long, and who your on-site contact is.
Warranty, in writing
Separate the two warranties that matter: the manufacturer’s warranty on the window, and the installer’s warranty on the workmanship. Ask how long each lasts, what voids them, and who you call if a window leaks in year three. Get it in writing. A verbal ‘we stand behind our work’ is not a warranty.