Start with the people who will load it. Count the people in the house, the meals cooked at home in a typical week, and whether the machine will run every day or only for large loads. Write those answers down in your own words. They are the planning input, and they matter more than any headline capacity figure.
Measure the opening, not the box
Take the width, height, and depth of the cabinet space the dishwasher will fill. Check the clearance above the front edge so the door can drop fully. Check the path the door takes when it opens, because a corner counter or a narrow aisle can limit how far it swings.
Check the rack against your dishes
Load the dishes you actually own. Set a large pot, a sheet pan, and your serving bowls beside the opening, or bring them to the showroom. Look at the upper rack for stemware and the lower rack for pans. A layout that looks right in a catalog photo may not suit the way your household cooks and cleans.
Check the utility path
Ask where the water supply will connect and whether the existing valve sits in the right place. Ask how the drain will connect and whether your local plumbing practice calls for a high loop or an air gap. Ask which circuit will serve the unit and whether the outlet under the sink is the right type. These connections decide whether the job is a swap or a rebuild, so get the answers in writing before anyone orders the unit.
Ask these before you order
Ask the seller what the listed capacity means and how it is measured. Ask whether the warranty or service terms change if someone other than the seller installs the unit. Ask who removes the old unit and who hauls it away. Ask whether the trim, the toe kick panel, and the cabinet face are included or left for you to supply.
The dishwasher page on this site sets out typical service life and when to plan. Read it once the fit is settled, so the planning date is attached to a unit that already matches your room and your household.